First off, don't fire all but three players, and dont dress three players per game. It's really just a disrespectful way to play the game. Second, while i wouldn't immediately hire a level 4 trainer in your situation, i think it's important that i point out two things to you: Potential and Trainers.
The potential of a player does not have any bearing on whether or not they will be an effective player for you in D.V or D.IV in the US. Potential merely explains to you the point in which your players will not be training effectively anymore. There is a massive help thread stickied, and on page 3 or 4 it tells you what each potential caps at salary wise. In addition, there is a training simulator which can give you an idea of what a lvl 4 thru lvl 6 trainer can do for you.
The trainer that one has does not affect whether or not a player can reach his capped status. What the trainer does is he can improve a player faster at a higher level. The reason why Alloy doesn't value this is because he doesn't see time as a value of money on trainees. However, many times people train 2-3 guys and have issues remaining competitive, starting sometimes in D.IV. If i can buy a level 3 trainer and spend only 6-7k more but win more games while training and thus bring in more fans, I can make the 6.5k weekly investment pay itself off. I think in your case, it's still worthwhile to get a level 2 trainer immediately as it's such a salary efficient move for the amount of speed you get out of training. If you got overzealous and picked up a level 5 or greater trainer, i'd go ahead and cut them after friday's training.
On your team, the only trainee i see right now worth it is Ade Casper. I have no way of knowing what he has so i can't really tell you if he is truly worth it. The guy you purchased from Hong Kong doesn't have the potential to be a good trainee target. A rule of thumb for me to evaluate talent is to count the skills. each rating of each skill has a number association. If you add up the 10 skills (dont count stam/ft), 47 is a good number for a starting 18 year old at the beginning of a season. Your player is certainly good at some of the guard positions (Casper) given his high salary, but might still have flaws that make him a less than stellar trainee. The 18 year old Allstar is close, but long term i dont think training a big and small is what you'd want to start with. the 21 year old MVP has likely missed roughly 35 pops minimum, but the MVP might still get buyers, so i would list him.
I would probably keep my roster close to 10-11, and learn how to manage game shape. Players play better or worse than their salaries based on how good of shape they are in, which is based on the minutes played between saturday and thursday of the previous week. The goal is to keep your primary players at an 8 or 9 GS as much as possible, and hopefully keep backup players at a 7 or better for the most part. I suggest when we get to three games a week next week (2 league, 1 scrim that you should always schedule), you should try to manage your starters to hit under 70 minutes per week, but above 48. Usually it's a start and two backup+mopup, 2 starts, or all 3 spots and then a backup role in one of the remaining two games that nets you pretty close to your minutes you need.
If you need any more help, you can pm me or sign up for the offsite and get on the USA chat (http://s3.zetaboards.com/BuzzerBeater_USA_NT/index/). I'm on there quite often too.