Poison Sacs is always on.
While all Poke-Powers are optional to be used, Poke-Bodies are always on, all the time, as long as the requirements to use those Poke-Bodies are met. Poison Sacs has no requirement, so it's just on from the moment you play Toxicroak. You don't have a choice in the matter, it's just always on.
Some Poke-Bodies only work while in the Active spot, such as Abomasnow from Mysterious Treasures:
http://pokebeach.com/sets/mysterious-treasures/19.jpg
I believe there are some Poke-Bodies that can only be used while that Pokemon is on the Bench, but I can't find a recent example of that.
You are right in saying Telepass can be used only once a turn no matter how many Gardevoir you have in play. However, if you had, say, four Typhlosion MT in play, you would be able to use Firestarter 4 times because there is no restriction listed on that card saying you may only use one Firestarter Poke-Power a turn:
http://pokebeach.com/sets/mysterious-treasures/16.jpg
So, to summarize, you may use as many different Poke-Powers you like during your turn. Poke-Bodies are not used, but rather always on.