I am having trouble with screenplay too - I have to write dialog, and I'm not good at that - so I am no expert.
I did have to make a series of 5 minute films for television, and the first one came out at 8 1/2 minutes! It was heading towards 10 minutes, but I cut out a couple of sequences before shooting them. But I would have had to re-shoot the bits I already animated to cover the key points in less time, and that was not possible. So it stayed at 8 1/2 minutes, but I couldn't get away with that a second time. So I had to learn to identify the essential parts of the story, the shots that could not be left out. The story basics, like who is your main character, what do they want, what would get in their way, how can they overcome that? Do additional characters or scenes really help to tell the story, or do they distract the audience from it? I started with those essential scenes, then if I was under 5 minutes I could add just a couple of my other shots if I still felt they would improve the film. A lot of the really cool stuff I wanted to shoot had to be dumped, because the story made sense without it.
Your story might not be too complicated for a 7 minute animated film. I can't tell since I don't know what's in it. But, I found that could tell more in 5 minutes than the average live action film, because with animation, you get right to the point. (It's too slow and tedious to spend hours animating unnecessary seconds of them not doing much, unless the focus is on another character and they are doing something important.) You don't have the puppet take a full minute to pour a cup of tea, you do it in 3 or 4 seconds, cutting right to the key action. Or, it might be too complicated, not because of running time, but because there are too many locations, characters, and sub-plots, and if you rush through them it gets hard to follow. Generally, leave sub-plots for novels and feature films. Short stories and short films should have a single clear narrative.
Have you storyboarded your script, to see how it comes up as visual storytelling?
6 locations - just looking at some of my 5 minute shorts to see how many. One, Cell Animation, had one core location, the inside of a prison cell, that took most of the time. But I started with a pre-title shot of the character as a baby, drawing on the wall with crayons, just to establish character. Then as a young adult on the street where we see how the character got into trouble painting graffiti everywhere, a courtroom, but that only took 2 seconds or less with a closeup of a gavel hitting the desk, a prison corridor, the mail room where cakes are examined for files, the cell, inside a jungle painting on the wall, then a shot in a monkey enclosure at the zoo. That's 7 locations. (Although, 3 of them are in the prison.) I could have done an exterior view of the prison building, but found I didn't need it. I could have left out the baby shot too, if there hadn't been room. All of it fits into 5 minutes.
With yours, the core idea seems to be how two very different people learn to work together. So you need them before the mansion, maybe disagreeing, and arriving there for some reason. Then perhaps 3 monsters to deal with, where they get better at using their own strengths in combination with what the other one can do. That would call for 3 rooms. And wherever the dead egyptologist is... well, dealing with him could be the fourth and final battle. Then the moment when they have cleared the mansion and realise they are pretty good at this. Then cut to business card, office, or shop front where you see they have gone into business, and roll credits over that. (For the next episode you could start there, with a visitor asking for help, or start with the next evil house where someone flees it, then goes to see them.) So let's see, characters somewhere, that's one location, they might be in a car that breaks down outside the mansion. 2- The Mansion exterior front door. 3 - Interior main entrance hall and stairs going up. 4 - the library. 5- dining room. 6- evil egyptologist's secret lab in the basement or crypt. 7- exorcist office. Ok, that's 7, but some don't have to be seen for long. Probably also need corridor where they go to different rooms, but it can be re-dressed with different props, paintings, mummy case or whatever to look like different parts, so that would be 8. (Same with the rooms, you can alter one to become another one.) But some aren't seen for long, so that shouldn't be too many. I know I don't have the details right (or possibly any of it right), but that's the kind of structure I can imagine. You'll probably have dialogue to get the sense of their personalities, and how they are adjusting to working together, so that would take up some time, which is why it would need 7 to 10 minutes rather than 5 or 6. It seems do-able to me. You will want to have some fun with the egyptologist's schemes, maybe give him a henchman to explain his plans to ( a mummy?) so the audience gets it too maybe. And there is probably a lot of inventive detail you could put in, but that would risk making it too long and involved, and holding up the main thread of the story, so you have to be selective. Which ones tell us the most about the villain, or the heroes, and advance the plot?
Great topic, and great replies from Nick.
I would add: there's a good book called 'Ideas for the Animated Short' which has lots of tips.
Also, once you have come up with what seems like a good story, ask yourself how you can convey the necessary information to the audience, preferably visually. What I mean by this is that you have to show your main characters. One way might be for them to stand still and tell each other about their qualities and flaws - but this would look terrible: there is nothing worse in any drama than having characters stand around telling each other stuff they already know. So you have to find a way to show these things, through their actions, and let the audience make up their own mind about the characters. For example, I have a character who is rich and greedy but fears that his soul will go to hell for all his wicked deeds. My solution to show this is to have him counting his money then fall asleep and dream of being dragged to hell by demons. To show he is hated, a piece of paper wrapped round a stone comes crashing through the window, jolting him out of the dream. The paper reads 'You will rot in Hell'.
One essential rule about short films and stories is that you need to set everything up at the beginning. This sounds obvious, but there are lots of examples of stories where something entirely new gets introduced halfway through, rather obviously to get the author out of a hole. In your story it could be some super weapon, never mentioned before, that suddenly appears/gets found and is the means of getting rid of the baddie. This is bad storytelling, and audiences spot it a mile off. The clever writer allows something like this in the first draft, then goes back to the beginning and plants the weapon somewhere without drawing too much attention to it. The really clever writer gives the heroes a bunch of weapons that look like they will be hopelessly outgunned at the start, but later are used in a cunning combination to win. So having everything necessary (and no more) for the story there from the outset is really important if you want the audience to get that sense of completeness at the end.
You can start this type of story either by posing the problem or the hero character first. Either you show some wicked deeds (Gotham City terrorised by the Joker) that force the reluctant hero into action, or you show the characters and then they go off to find a task to complete. Either way, Nick is right that you can show quite a lot of information quickly, but you must stick to the essentials and strip away anything that does not serve the story - be ruthless! Doesn't matter if you have a brilliant scene, if it does not help the story to move forward, it must go...
I would recommend reading Exploring Visual Storytelling. Here's an Amazon link. I recently finished it as I've been working on turning my loose ideas for my first short film into an actual script. The book aided me in figuring out areas where my story was lacking. Surprisingly, it also gave me confidence when I found out that some of my ideas already fit into the foundations for storytelling. It's a short book, but I found it gets right to the point. The last chapter also has a multitude of questions to ask yourself if you get stuck in the process.