I know that sailfin mollies are usually presented as tropical fish, but upon doing some research I found that their range extends into North Carolina...which is definitely NOT a tropical region. Thus, would wild-type sailfin mollies do well in an unheated tank as long as breeding was not a huge concern? (I was going to boost the temperature with a heater for part of the year to encourage spawning)

Yes, they do fine and will spawn in the upper 60s.

Thanks! By the way, do they need salt, or will hard freshwater work?

Freshwater is fine, but ime you get better color by keeping them salty. Prettiest ones I've ever seen were kept in full strength seawater.

So long as you don't leave your windows open at let your room temps get much below 60 F, basic room temps for most people.

They can take low temps but need to be eased into it. Drastic sharp drops shock them and even though they sometimes seem to recover, later they do that shimmy thing and that's all she wrote.

I killed a trio recently in Florida when I took a bucket that had been in my hot car and brought it into an air conditioned motel room. Rooky mistake I know, but I was afraid to leave the bucket outside the room for fear the feral humans in the neighborhood would bother it. Eventually I said to heck with it and kept my cooler out on the terrace. Never saw it happen, but no doubt several thirsty [more or less] human critters peered in hoping for free beer.

Salt is not needed, hardness and minerals are. The ones I collected recently in Pensacola were in pure fresh and were the most unbelievable colors I'd ever seen. SO far, at home, I have only obtained about 75% of the color I saw under the strong Florida sun, but they're still real Purdy.