First of all, you can't train 6 players at full efficiency.
Getting 4 would be optimum at best, but that's also only for phase 1 where you do One on One forwards. After that 3 is the max, and 2 is optimum. Meaning you sell off the other 2 players.
For trainer, you would at least need a level 5 or 6 one. Don't go for high salary ones unless you have a lot of money in the bank to cushion your salary. My crazy hypothesis is that if you get a level 7 trainer who's salary is almost 100k for the price of 1k, it would take 9 weeks of training before breaking even with a low salary level 7 trainer (iirc they usually cost 1m+?). Not a long term solution tho.
As for training, if it's guards, I always start with One on One forwards, until DR and HA reaches 12, then proceed with OD til 12, and then PA or JS/JR depending on what I'm building.
Always go for single position training.