[SmokeRulz]

Yes, Ranks correspond to your Level. Add up the Ranks of your Powers, and as long as they're equal to or less than your Level, you're fine.

You can have as many Powers out as you want, as long as their combined Rank doesn't exceed your Level. You also can't have multiple copies of the same Power, only one of each.

And yeah, you can play them (if possible) as soon as they're acquired, but if you decide not to, then you can only play them when it's your turn.

Note that these rules apply to all Munchkin sets that have Powers, not just Bites (those would be Super and Zombies).