Let's try this...

[Haine]

Is there a way to see what kind of sprites already exist in the game?

Aside from looking through 4407, you're basically just gunna have to ask.

I'd also like to have a basic sprite for the new walls, if I decide to try my hand at themed worlds, and for the AI.

I'll post 'em when I wake up, sure.

Could a reward skin replace the base sprite of a human or cyborg?
Like the lizardman mutation or the automaton model so that other costumes would layer on top of that.

This is currently how the automaton reward skin for borgs works, yeah. I'd have to look into how to do it for humans but it's certainly possible there as well.