Platy? I wouldn't. They are a bit on the messy side and are not loner fish, but community fish.
Realistically the only fish for that tank is a betta, prefferably with plants.
The problem is still that with such a small space the temps can flucuate radically. But if the tank is generally hanging out in the mid 70's a betta should do fine. Lots of water changes will still be needed, even with a filter. Plants will help if you have proper lighting.
And never stick such a small tank near a window as the temp changes are really high.
Seriously though, a 5 gallon tank costs practically nothing at most stores, and a betta in a planted 5 gallon is awsome in comparison, plus you already have a filter. And a 5 gallon would allow for a small heater.