This is harder than one might think. Ali could not join, because it would bring up conflict between littlefoot and his friends (I mean, if another character was being added, they would be even older by then, right?) And I think it would be unfair to the others cuz they don't have any 'girl/boyfriends'.
Or even great friends, like Spike and Tippy. Tippy would not work well because Ducky would get jealous.
Mo definitely would not work because his very own way of movement is quite limited.
Doofah's a bit too big and er, different. I would think the smaller one, the green one, would work best. BUT, he can't really talk (like Spike), and he leads a WHOLE HERD. So erm, no lol.
And finally, Guido would actually be the most fitting of the characters you listed because he would bring a different side to the gang, but he's a kind of awkward...
So I chose Someone else. I would think a character that can conflict with Ruby would be interesting.