At school there is a 33 gal tank with 5 clown loaches and a single bala shark. The bala hangs with the loached and is nervous as hell because the tank is too small, doesn't have enough cover, and he has no one to swim around with. As I have been told bala sharks need to be in groups. Single bala's have jumped out of this tank in the past and died. This one, since the holes in the cover are now filled, bangs into the glass whenever someone walks by. I have to keep insisting to my dad that I do not have room for bala sharks and he keeps telling me to get "just one"
As I see it, having just one bala in a small tank is wasting your time and money, because you are going to have to keep replacing them
The loaches, also don't do well in that tank at school, they are stunted, and some of them have these nasty black spots on them.
I stand by kuhli loaches though. But they need to be in groups. I started with 2, to see how they'd get along with my puffers, but I never saw them, so I bought 4 more. Safety in numbers I guess, because there is always at least one swimming around my tank where I can see him. Its a 10 gal.
As for the platies, aren't they livebearers? And in that case, unless you keep all the same sex, the numbers should be 1 male for every 2-3 females, because males can exhaust a female by harassing her. I keep guppies at 1 male to 3 females, and if I had platies or mollies I'd do the same with them.