Melafix contains something like 1% active ingredients - it does very little beyond prevention (and I like the smell) and should be harmless.
Have you tried asking your LFS for a special order or to make an exception (it always helps to make a huge fuss - or try suggesting they've actualy picked out two male or something - most don't have a clue). Otherwise, you can always try a different fish store/pet store or even order online.
It is entirely up to you whether or not you get more dwarf gouramies - just keep a close eye on your fish if you don't so you can spot things early if they go wrong.