You can kick everyone but 3 players, but don't do it, it's one horrible tactic, and I would argue it's not even financially good.
22)You don't need an advanced trainer, competent will do. I'm using one too.
Take a look at my example(I started last season):
1st season: Just expanding the arena, invested about 3 million
2nd season:
-Bought a few players for about 700k, just to reach the salary floor
-Continued with developing the arena, about 800k invested so far, another 200k upgrade planned for tomorrow
I'm currently at 15k/1500/500/20 which is already bringing me about 260k per match(could go up to 300k with better prices)
I plan to finish it by the end of the next season, it will take 3 more million.
After I finish it, I should be able in cash in about 350k per match.
Anyway, with only a couple of seasons of patience, I will gain tremendous financial power, which basically means success in this game.
I would advise a similar strategy to you, first season arena, learn to manage game shape etc. second season get some cheap players to start competing, just reach the floor and keep expanding arena. by the start of the season three, you should be in a very good position