So, after working on 3D modelling for a while, I want to sharpen my coding skills with c#, so I was thinking about using pre-made assets and making a game with just my code. Has anybody tried this, and how did it go? Any tips?
pre-made assets are great - use them if you can. Be sure to match styles though, so they don’t look too different. 
I have a side-project which is sort of doing that. Not sure if it’s going to turn into anything I want to distribute (mostly a testbed for whacky ideas), but for the most part I think it’s doable. The one thing you should spend a little extra on is custom player models, though. At least some texture modifications for a unique look, but preferably contacting the creator of some nice assets you have for further custom work.
Of course some types of assets are quite universal. As long as everything you have matches in style, a tree is a tree is a tree 
Well, I didn’t really plan on releasing it, mostly just a place I can go to to put some random code and practice, have a place full of random things.
Yeah the problem is that its not going to be one unified style, but its quicker and cheaper and maybe you can hire an artist to tweak some of things to make it closer to a particular style. I think the bigger the project the harder it will be to do because its going to require more custom assets, you can find zombie assets but no hero or arenas.
If you dont plan on releasing it then its fine who cares what it looks like.
I do it all the time, works a treat. If you are worried about styles, just use them as placeholders, complete the game, then if you think it has good game play you can choose to change all the assets or keep them as is.