You are able to put any character that has a itch.io page into a fighting game; 

And yes, it includes games that have releases on other sites too like deltarune and fnf.

 You can also add characters from your games,

But you cannot add characters such as mario or crash unless it's the version that comes from a itch.io fangame, For example, Sinister squidward (R.I.P)  is allowed, but The canonical squidward from the show.

 What would be your roster?