I have had mice in the past. They can be a real delight; very animated and fun to be with.

They can be hard to sex and recommend only getting one or you will have way more in no time; there are risks of overcrowding and nowhere to go with them. It is not the case that two females need to be kept together. I would definitely recommend only getting one especially depending on the size of the tank. If the shop is good at sexing, a male is preferred as they are more docile and accepting of people, with fewer surprises down the road of little ones.