if you are swapping big men to sfs, why not train the one sf and a couple of guards along side him?
the guy above is a guard trainee (a bit tall, but not the end of the world) rather than a small forward especially given his ID.
rather than buying another trainee now see if there is anyone else on your roster that could benefit from outside skills.
even if it is just for the rest of this season why you save up cash, invest in arena, and then next season get a better trainee.
so, looking at your roster it looks like you bouught the 6'4'' guy. There is your 23 year old Star SG that could maybe take a few more pops especially if he has a weak skill. i also see there are 22 year old big guys. these might be worth rounding out outside (JS and PAS are needed by all players). might be worth considering each scrimmage game when playing a big guy at PG or SG to train wont matter.
I'm not sure your issue for long term development is having the "right trainees". you probably have plenty of material to work with in the short term to make a bit of cash and improve court performance. You could look hard at managing your wage bill and other expenses/ticket prices to gve you the ability to increase your arena more quickly.
in couple seasons your trainees will be doing well, your other guys will be rounded out and youll have a pot of cash in the mean time through having grown your arena :)