You dont need to open spellbook to use them. If your first bar is full, once used a poly skill, it will be availble in the second bar.
I think there will be more sub skills based on the animal forms (Horn,Wing,..) on full release so the transition you suggested wont work.
Hovering to that Poly skill and it appears all the available sub skills of that form in a new mini-bar would be neat but the game is close to release so it probably wont be thier top priority.
