I always recommend a heater with bettas. You should keep the heater set at 26 or 27 degrees C. Even if the room temperature drops only a degree or two at night, and causes the water temperature to drop, the betta will probably get stressed. If this happens every night over a period of time, the potential for stress/disease is greater.
I wouldn't try a male and female betta together as tankmates. Either betta, male or female, will attack the other. Providing cover for the female may not work because she may be just as aggressive as he is. Usually for spawning, the two are introduced through a divider, and only kept together for a short time to spawn, and then separated, as mr86mister says. So I wouldn't try to see if they can live together as tankmates.