My guess for why Juste's in and not Christopher would probably be because at the time of PoR's release (3 years before Adventure ReBirth), I doubt many CV fans even knew who Christopher was at that point and hey, Juste was still pretty relevant at the time, so it makes sense that Igarashi would include him.
If there were to be a 7th member though, it'd probably have to be a Belmont (I think the in-game description mentions that it's the most powerful of the Belmont clan, not so much Castlevania heroes) who's canon according to the Igarashi timeline.
Let's see...err...we have Soleiyu and Desmond left.(Is Desmond even canon? I'm really not sure.)
Not exactly the most well-known of the clan.
I mean if we were to talk out of all the Belmonts, sure, Julius certainly counts as the 7th great member, but he's the last canonical Belmont, so that probably wouldn't work.