I have two females; that way, while one momma has her babies, the male rat can be with the other female. Actually I am raising another young female to be a third mom rat, so he will definately never be alone. I think that they actually can get depressed when they're alone. But never try to put a male in with another strange male; they almost always fight.
I only use 10g tanks when a mother rat has her babies. They really do not move around much at this time, spending most of their time on the "nest" they create, suckling the babies. But all non-baby-having females are in the big, under-the-bed type rubbermaid containers and that gives them enough room to run around with the male.
Make sure you give them something wooden to chew on. They need to chew, and if they have nothing, they'll resort to gnawing holes in their water bottles!