Their are people on this site who sell plants and they would be a lot cheaper than buying retail. Also their are people selling on trademe and they would also be much cheaper than retail.
They may have been moved to another site, but yes be wary of the male. Also after about 6 weeks they may want to spawn again which means they will see the previous fry as a threat and eat them.
Platy's prefer temp of 25c so maybe 30c is a bit high for them to tolerate and this method takes 3 weeks to be successful. Anything with acryflavin,Quinine sulfate or malachite green will act as a cure.Also when you see the white spots this is the terminal stage which precedes death. White spot is usually caused by poor conditions.
Having googled 'aquatic community' datnoids are very sensitive to nitrates, regurlarly go on hunger strike and fast eating fish in the same tank would help.
405 should cope without doubt. 7 months may play a part with a small tank however the only advice I can give would be to clean the filter and do regular water changes.
They do not have much colour but seem to come from a green strain. Some of the males are apparently colourfull but they were few and far between if my memory serves me right. The hunters looked very brown as if they had been dragged through the mud by their hair.
I use a bilge pump connected to a car battery on one of my tanks. On another I have a hose permanently connected to outside tap tee'd off with a valve. Stick hose in tank and open valve and gravity does the rest. To fill I just close valve and turn on tap. 10 mins to do a 100L + water change.