Problem is the tank size, 60 liters (15 gallons). This is not sufficient space for a group of dwarf neon rainbowfish and yes this is a shoaling species and should have at least six with more being better, but as I say, there is no room here. I don't know how long you've had them, but if not long I suggest you re-home the rainbowfish.
Five platies and six guppies are more than enough for this small a tank. Be regular (weekly) with a substantial partial water change (60-70%) and feed sparingly.
I hope the livebearers are all male, no females present, or you will soon have more fish than you can manage.