Fanon=What we've speculated based on that
Fanon may or may not become canon at some point in the future--not every speculation is wrong--and it's fun to play with sometimes, but it Is. Not. Canon.
There is nothing in canon (unless JKR has updated her site on the topic sometime in the last eight hours) saying that no Black before Sirius was in Gryffindor. Nothing. Nada. The notion that Andromeda couldn't have been in Gryffindor is pure fanon with no basis, and it wasn't fanon that I found interesting, so guess where I put her? And guess where she's staying until J.K. Rowling says otherwise?
(Well, actually, I probably wouldn't retcon things already up even if JKR contradicted me, that stuff's finished; I just wouldn't write new stuff contradicting new canon.)
Lily being a prefect is sensible fanon--if JKR remembers that she was head girl--but it's not canon, so if you write a story in which some other Gryffindor girl is the prefect in their year, I'm happy to entertain that idea. Draco being actually a nice guy in a bad situation is fanon, but if JKR goes that way, I'll promptly write him as such. Just...