Squall being Laguna's son is heavily implied throughout the game via Ward and Kiros' NPC dialogue, Raine's backstory, the Moombas mistaking you for him and so on.

Rinoa being Ultimecia is not implied at all. Ultimecia's hatred and evil is mentioned, and obviously she must have inherited Rinoa's powers (via someone else!) in her own timeline, but if Ultimecia is Rinoa because of that, then Rinoa is also Adel and Edea, because she inherited their powers. What this theory then says is that Ultimecia is Rinoa, Adel and Edea, as well as the other eleven Sorceresses you pwn on the way through time compression, plus plen-tee of offscreen characters.

Basically, the fact that one gets heavily implied whereas the other is only generated by vague allusion is all the evidence I need to say no, I don't think Ultimecia is Rinoa. If they intended us to work that out, they would've filled the game with obvious clues like they did with Squall and Laguna.