They sound pretty much full grown to me, i think there are two main things to consider;
a. what are their genders? it may be worth exchanging some of them for different platys at your lfs to prevent them from inbreeding which can have some serious side effects in the long run.
b. do you want to continue breeding them? if so it may be worth leaving some space in the tank for future generations of fry to grow up in.
I would personally go for corys, peppered, albino, bronze, adolfo and panda corys(although panda corys are about as hardy as neons) are very good. You could easily fit 4/5 in there which is a good number for them.
Do you have sand or gravel though? if you have sand, your tank is perfect for corys but they don't like gravel as it can damage their barbs/whiskers.
Other fish which would look good are various tetras, neons are my favorites although alot of people seem to have problems with them dying although personally i have never had any probs with mine; keeping constant good water stats is essential for their survival.
You could also try other livebearers in the like guppys too.
