Smash's Puff is a female though based on her alt colors including female Trainer hats while the Male Pikachu has male trainer hats.
I'd say that's tantamount to saying the handful of legendary Pokémon that happen to have masculine voices are male, even though Pokémon like that, such as Mewtwo and Darkrai, tend to be genderless
palette swaps aside, you are right about Pikachu being male since it lacks a notch at the end of its tail