Endlers need current. If you have a sponge filter I would suggest at least one bubbler to help oxygenate and to give the tank needed water movement.
As far as number, you could have a dozen or so with no issues even if you have other fish. Habrosus Cory make good tank mates and pick up leftover food. I also had my male Endlers with a Betta.
I prefer a Enheim Jager heater. I'd go for 75 or 100 watts. Better to have too many watts then not enough.
Aquarium Heaters: Eheim Jager Thermostat Heater at Drs. Foster and Smith