I've been checking threads and videos but I'm still not sure.

How does the ST work? Should I use a Pokémon I don't care for during the ST (soccer ball game) to win good training bags, and on the training bags use desired Pokémon?

Example:
Use a Bidoof for the soccer game, win a Attack L-bag and then put my Tyranitar on the Core training with the bag?

How long do I need to tap the bag to "finish" it?
Or can I use my Tyranitar for the soccer game too?
I want to be able to max out, and doing S bags instead of L bags takes me longer time, but there's no "stats loss", correct?