Ok, here's my thought:
I generally like your idea, Ducky.

When they start from a huge discord, and escape a valley with such discord in their hearts - this is good for me.
And as I said, I like mystic, so their species changing, feelings about trip to some place, and possible further events during their journey - this is what I like!
Still, we need come to agreement with species, because "Spike is bellydragger because it is fun as hell", or simple switching species or turning into species opposite to original one - that's not good, me think. As I see - during, or at the end of journey, they should realize why exactly they changed into what they are now. I think the most of you just want to see Littlefoot as hunter and Chomper as leafeater and all others as swithed species or opposite species, but I think it's not the right start - everything should be logical, reason should be the same for all gang members, and their changes .
I think you understand me.

Anyway, if I would be alone with this point, I don't mind, I would easily accept your list of species.
Oh and I think they should be older. Teenaged. Old enough to fight sharpteeth. Because that would be awesome.
I agree to see gang more grown up, from couple to five years old older. Plus, around this age, kids' temper starts to change, so this is where discord come from.
But ahem. Five years and 0 years old kids "easily" kicked ass of the possibly most dangerous Sharptooth on the planet. And it was OSOM!

So I don't think they should be very elder teenagers or something like that. Plus, even if they would be old teenagers, I don't think she would do something in direct fight with sharpteeth like velociraptors - gang are still too young for this.