Really, the only requirements for theriomorphs is that they have the powers of Beast Change, Echoes of the Beast, both tying to an applicable animal form, Human Form, and a minimum of 2 points in some Inhuman or better skill(s) from the Speed/Strength/Toughness category, or the Minor Talent or Creature Feature categories. Again, with the requirement that any powers taken have to be applicable to the 'animal' form. As such, regardless of the Refresh level of the game, a 'weresloth; ( a 3-toed sloth being one of the slowest moving mammals) is not going to have Inhuman/Supernatural/Mythic Speed. Nor would a 'wereslug' with the Claws power really work, since slugs lack teeth or other rigid/chitinous deposits.
Now for someone playing a 'werewolf' the wolf-form could progress as the player acquires Refresh, going from an animal which resembles Canis lupus to something which has a greater resemblance to the extinct Canis dirus, more commonly known as the Dire Wolf. What could be interesting for those playing ursine were-forms, would be to start playing a were (black) bear and gradually grow as Refresh is earned/spent to playing a were version of the Arctodus simus giant bear. This would make the power Hulking Size applicable, at least for the animal form.
Just as long as the powers chosen fit with the selected animal form, advancing the powers up the relevant tracks should be fine.
