The people translating the profiles are members of BL and most of the datamined stuff about GO comes from there as well.
Regarding Prototype, it is canon as far as Nasu is concerned, considering that he wrote it himself. It just occurs in a different timeline than the other works. Even mainstays like FSN and FZ aren't really congruent in terms of timeline and canon, as some FZ stuff contradicts FSN and so on. Nasu's stance is basically everything that doesn't fit in one timeline is in another timeline and so on due to Zelretch's Kaleidoscope, since he can view parallel universes.
GO Servants have varying flavours. There's the normal ones, which have appeared in works like FSN, Apocrypha, Prototype, Fragments and so on. There's the mildly silly ones introduced like Orion/Artemis, Bonny/Reed, Nobunaga and Okita. There's those that were initially scrapped like Kintoki, George and David. And finally there are the absolute silly ones like Santa Alter, MHX and Edison/Captain Leomon.
About the actual additions, could we perhaps start with Apocrypha Servants? I'll make a list here for the ones missing: Atalanta, Shakespeare, Semiramis, Spartacus, Astolfo, Avicebron, Jack, Frankenstein and Shirou Kotomine.