Am I allowed to create a signature shop going off-topic of toribash?
(By offtopic I mean that I dont only focus on tori's in the signatures)

That's still considered art, you don't need to make toribash related stuff.
I think the right place would be the Art/Textures thread.
You should make your request shop so people can request anything they want, or just simply check the request area and do some simple ones there.