oh dear platty 2 doesnt sound good.
the hump back could be down to internal parasites, i have no experience of sterazin but understand it is good for both internal and external parasites, i think a full treatment course takes about 2 weeks??
its probably a bit of a gamble where you house & treat platty 3 now but hopefully he will be ok
as for your rainbow - firstly how envious am i? i think they are stunning fish but have never seen any for sale - i cerainly dont think lighter feeding will hurt them for a while, you could try some small floating food rather than flakes i ave heard of them playing with peas tossed in a tank for them...
good luck
edit: 3 three posts prior to mine were all written while i was typing....