It's a single frame between each powerup right? Like, going from wheel to hammer or mike to wheel is a single frame right?

The timing you have to select the power varies about 4 frames in KSSU, not sure if the same rule applies here but I would imagine as such. The wheel as it slows down also has the same button press window for powers despite the slower speed, making it harder. Even so, most powers you'll need to mix for can be done by holding b + down + a as you mix.