You are right that the fluorescent lights don't give off much heat. That is what you need for starting seeds, so that you can get the seedlings right close to the light without burning them. Having a couple incandescent lights somewhere else near the plants, but not right next to them, might help keep the temp up a bit without the safety issues of a space heater (though personally I think the ceramic heaters are quite safe and I would use one). Besides safety though, a space heater is a big power drain. I already worry about how much power my seed starting operation uses with all the lights and two heat mats.
Anyway, a couple regular bulbs might help in a small enclosed space - especially if the space is insulated. And you are right, as long as they are not freezing, the soil temp is most important. As long as you can keep the soil well warmed up, the cold air will slow them down a bit, but not stop them. And they will be better adapted when you put them out. Be sure you get them off the heat mats once they are well started, so they aren't too coddled.