I think that their personalities are different, but there can be found similarities, sure. Most of the similarities, though, are probably going to be influenced by either writing conventions or, more importantly, the roles that they have to fill in the party.

Quistis and Lulu, for instance, are different people, but they take on some similarities in large part because of their 'older sister' sort of roles.

I think that putting them into trees is probably going to oversimplify their personalities, too. For instance, Yuna, Rinoa and Tifa are very, very different people--and I'd have to disagree that Tifa is tougher than the others in that tree, if that's what you meant, because from what I remember, Tifa isn't actually very tough at all when it comes to personal matters, though it might not be fair to take her dealings with Cloud as an indicator of that. (Edit: Unless you mean she has a different sort of toughness, which I'll go along with, because she does.)

You can group them, but it's more 'party roles' than 'personality types', and even that doesn't hold up universally. Squall, Cloud, and Tidus make a strange group in therms of personality, but Selphie, Rikku, and Yuffie don't really fill the same role at all in the party.