If I evolve my Shroomish into Breloom (which has the Spore attack), can I use Amoonguss's "Bursting Spores" Ability?
Yes, because you have played a Pokemon that has the Spore attack from your hand during your turn.
Bursting Spores




x2Bursting Spores
Whenever you play a Pokémon that has the Spore attack from your hand during your turn, you may leave your opponent's Active Pokémon Asleep and Poisoned.

VenoshockIf your opponent's Active Pokémon is Poisoned, this attack does 70 more damage.
If I evolve my Shroomish into Breloom (which has the Spore attack), can I use Amoonguss's "Bursting Spores" Ability?
Yes, because you have played a Pokemon that has the Spore attack from your hand during your turn.