Maria existed as a person in the sattk canon, yes but she’s a person of the past and died. I don’t know if she will be mentioned or not. She might be. She won’t make an actual appearance though.
Vanilla, Cream and Cheese -might- appear if I need more characters but at the moment they’re not planned. (but then NPCs are always needed so there’s a decent chance they might appear UwU)
Cosmo is not gonna appear.

Aaand no we don’t have a youtube channel but we might make one in the future to host whatever animated content we have such as a trailer and our intro. So youtube content might be a thing later on yes.