I'm going to disagree with Paul here and say that you can keep a sexed pair in a 10 gallon. It's not ideal, but I know a ton of tinc breeders in real life who just use a 10 gallon for their pairs, and I have 2 pairs set up that way myself.

As for the temperatures, my tincs get down to about 65 °F (18 °C) at night and up to about 75 °F during the day.