Fluorescent is not necessarily safer -- the ballast creates heat, and it is a heat source much closer to surfaces than a lightbulb would be. I've known a couple barn fires started by fluorescents, and several people who've had the little compact under-the-counter units start melting/smoking/smoldering... whereas I do not personally know of any fires started by, like, a LED light string or a 25 watt incandescent bulb.
However. Be aware that it is hard to winterize small tractors in cold climates -- the small volume of air makes it awfully difficult to maintain sufficient ventilation (to keep humidity and ammonia down) while not ALSO making it too freezy/draft at the chickens. It is not impossible to winter chickens this way but it is not the easiest thing on the chickens or on the chickenkeeper. Have you looked at whether there is any possible other way to overwinter your chickens, like in a shed or something. At the very least, it would help to pull the tractor right up alongside the house (or etc) so you get some thermal buffering and a good windbreak. Also that would be a shorter run for any extension cord you end up using (which is not terribly safe, although if you can ensure it is somewhere it will NEVER be tripped over or shovelled into and only use a single, HEAVY DUTY cord it is less unsafe than it would be otherwise, oh, and make sure it is rigged so that moisture cannot drift or drip into any plug connections)
Honestly I think you are going to have enough challenges keeping chickens happy and healthy over the winter in a small tractor that lighting them to maximize egg production is the least of your worries.
Good luck,
Pat
