I don't think the temperature would be as big of a problem as evaporation would. Losing just a small amount of water would throw
salinity way out of whack. Also, what would you keep in there except maybe a few mushrooms or zoas. If you really want small, I think there's a 3 gallon on Doctors Foster & Smith site for about $40 that would work alright. I'm thinking about getting one for myself to sit next to my 75 and will use it to
acclimate zoas and mushrooms for 2-3 weeks when I bring them home from the store, and also maybe to grow copepods.
Just do as much research as you can. I got my 75 gallon mid August, and I haven't added salt yet (though hopefully next week!)