Ideal Temperature Platy's And Tetras?

Black neons need 23 to 28 deg C; platies need 20 to 26. Guppies and endlers, if you want to keep those, can go as high as 28.

You need to aim for the 24 to 25 deg area.



Did you mean you want all male black neons or just all male platies?

25 deg is fine.

You don't need to worry about black neons breeding. Even if they did spawn the eggs would be eaten as soon as they were laid with both adult black neons and platies in the tank. And any fry that managed to hatch would be quickly picked off.
It is also next to impossible to sex fish like black neons in a shop tank, and the shop worker would probably refuse point blank to catch a particular fish from a tank full of fast swimming, identical fish.