not sure i can give great advice on scaling here because i was kind of making it up as i went, i started off drawing the zombies at a 2048x2048 canvas and drew at that general scale for most assets, except when i realized that drawing things relative to the entire screen (helicopter frame, results box, the sniper in the corner) to that scale would be wayyyy to big for my comfort so i drew those on a 3840x2160 canvas. here's an image to show how ridiculous the scaling was in comparison.

this ended up being annoying when i wanted to see how things would look in comparison to each other so in future i think i'll stick to drawing from 3840x2160 or double that, which will get scaled down for most screens anyways. in general i think you wanna draw big, it'll give you more leeway to scale things around even if ideally you don't, but particularly with the tight turnaround of a game jam you'll have to accept that it might happen. in this case since the slasher and enemy sprites are put on a 3d plane to be skewed slightly to the camera you can't maintain the original image quality there anyways. and if you're importing into unity make sure you're exporting as squares with dimensions by multiples of 4, personally i want to get more hands on experience with the game engines so i better understand how to make easier to work with assets but if you aren't working with the programming yourself, i'd suggest asking the devs for their preferences!

there's actually very little shading on these assets, just around the heads of the enemies so they're more readable from the rest of their body and behind the ribbon on the results screen. i'm honestly not confident with shading and it also saved our programmer time not having to apply shadows on everything, but i'm glad the style seems to look great without. i tried to design with distinct shapes and in mind so they could be read without a lot of detail. you can see this with the sniper where i avoid detailing the limbs by posing them out from the body, and the bat where the square body leaning back and forth and shape of wings angled in and out have enough change between the poses to convey flying and attacking in just two frames. use references!! i didn't know how to draw most of this until i looked at references for what i wanted. and worth noting that our programmer actually moved the sprites in engine for the dying and swooping motions, otherwise the enemies are just two frames of animation. the thick borders were also their preference, and i think it helped the assets pop from the environment without shading.

another thing, and this is just my preference i'm no expert, but i notice sometimes when art is scaled down to playable sprites the lineart looks too thin when you see it ingame. if you have time outside of jams you could do some practice sprites in different brush sizes to see how they look on a game screen and go with what you think looks best. i'd also watch out for assets having visibly different line widths from each other ingame, i'd try to draw anything meant to be on the same plane with the same brushes or at least being able to check them side by side to the scale they'll be on screen. i personally don't think lineart on backgrounds should look thicker than on assets in front (things further away look smaller) but in the end it's your own style so just try and see what you like. 

maybe the programmer can tell you more about pixel texture filter, if you're talking about assets looking kinda pixelated in our game. i don't know why they're like that, it probably has something to do with how it's rendered or set up in the engine.

sorry if some of these answers weren't as helpful as you wanted, i'm frankly not so experienced with all of it myself but i threw myself into the deep end with this project and you can see how it turned out (good i hope....) in general i think there's a lot you'll learn just by trying and seeing what you like, so just try things! and if it looks bad you'll be able to learn from and correct it moving forward.