Seems non-spammish enough to be a real first post
I'll see if I can answer all your points:
1) O.K. well you get 7 creatures (that includes the credit snake, and the expansion creature).
2) One creature per level (unless you grab a second controller, and use a different one for that particular level (a common way to make some of the trophies easier).
3) Completing a level unlocks the next level, and therefor the next creature.
4) Every creatures level has 16 'layer' screens, except the credit snake's level.
5) If you are exiting the game fully, the only thing that is saved is how many creatures you have unlocked, so finish a level to 'save' any progress. However trophies can be unlocked mid-level and at any time.
Hope that helps, feel free to ask anything else.