Not 6 trainees! That's the maximum amount of people that can recieve training in two position, which would be very difficult to do and not very profitable. You want 1-3 trainees. Two is probably easiest. One position train (e.g. Pressure for PG and PG only) because it's faster than any other kind of training. Training is possibly the most complicated part to understand about this game other than the game engine. Each player needs to get 48 minutes at the positions listed in the training. So Pressure for PG will train every person that played minutes at PG that week. More minutes = more training with a cap at 48+. This means if you're training one PG and one SG than you have to play them both at PG in order to recieve training. Then if you train Outside Shooting for SG you have to play both of them at least 48 mins at SG. Training only occurs on Friday, you can change training as much as you want and then all training for the week happens on Friday for whatever skill is currently set under the training page.
Arena - you want to be getting the most money from your arena, and the longer you have seats the faster they have payed themselves off and can start earning you money. I'd build a few lux boxes and also increase the others. Not a lot, you need moneyfor players, but enough that it helps your income. And yes, soon is good.