Well, actually you can't that easily... since you can only release as many players as you picked from the winning team. But after a little time, dormitory comes in and help you out.
For the Mii, I didn't train any yet... I was in an hurry, but I prefer to wait for the players I want to be in my team, since you need to choose 2 coachs for him... at that time Fowler and Wright-philips (my 2 best players were not good enough).
Now, I finally got my private jet (can play national teams now) and some luck with picking players, but still have to train my best players too... give them special or advanced skills can give the Mii more skills when you create him as a player.
The champion's road is just so great.... but it takes time! At least for now, I have a beginning of a great team if anyone wants to play in champion's league mode online! :) Gonna continue a little after I guess!