If Guise uses I can do that too! to use another hero's power, can that hero then use that power on the same turn?
For example, Guise uses Grandpa Legacy's Gung-ho to let Grandpa use a power. Can Grandpa use Gung-ho too?
The rulebook states a given power can only be used once per turn, so I would say not.
Did the hero whose power it is use it yet? No? Then sure, he can use it, too.
It does not say that a hero may not use the power twice, but that the power itself may not be used twice in the same turn. Note this is turn, not round, so as long as Guise can restrain himself from playing I can do that too on Legacy's turn, or any number of other power granting abilities can refrain from using it Legacy can use it on his turn.
Just have to organize you throwdown to not stop your allies from helping you rock.
So would I be correct in assuming that if say, Argent Adept uses Inspiring Supertonic on his turn on Haka causing Haka to use the power on Mere than Haka could use the Mere on his turn (because it's a new turn)? Whereas if someone has the Captain Cosmic card which lets them use a power at the start of their turn that they couldn't use the same power at the start of turn and then during their power phase (because it's still the same turn)?
Yep