Since you are from down under, you may be able to find Endlers. Another fish that is not popular here but is over sees is Heterandria Formosa the least killiefish, smallest livebearer in the United states. I have a 2.5 with a colony and I have a 5 gallon with atleast 15 swimming around in a heavy planted tank. The interesting thing is the tank doesn't look crowded, I have to hunt to find these guys. They are that small. Adult males are around 1/4 inches long, with females reaching a max of 1 inch. What also makes these guys great for small tanks is the fact the are from southern USA and will tolerate room temps fine. Tank can dip to 50 F, 10 Celsus, and the fish will be happy.
As for endlers, I find them to be smaller than guppies and just as prolific. Hence, the reason why we never see females for sale here in the US at the LFS. I have so many in my 15 gallon tank, that I am working on setting up a second tank for them. But a trio of wild form guppies or Endlers would be find in this tank.
As for platies, I think this way to small for them. I wouldn't keep more than a trio in a 10 gallon. I like to give my live bearers room to move around and this helps them to be more prolific and keeps them healthier.
So out of the choices, I would either go with guppies/endlers (one or the other do not mix), Heterandria Formosa , or a Killie pair of your choice.
jim