High 60s to low 70s if fine, and they can deal with temp swings in either direction.
not sure on the sexing them....Never really bothered to look it up. Seeing as there is literally tons of info out there a quick
www.google.com search should pull some answers for you.
You do know that the fantails can get as big as a grape fruit and the comets get well over a foot long right? I mention this because your 10 gal you have them in is in no way a suitable home for 1 let alone 4.
Edit: Sorry if someone else has already said something about that.