I would say 1.5 is too much for a 20L tank. I only keep up to 4 rats at a time in a 20L and its never for very long. That's just too much ammonia building up.

A 55 is good for that size group. Lots of room for them to move around. I use igloo houses for some of my rats. One of my momma's especially likes her igloo. She turns it upside down and drags it around trying to feng shui her nursing tub.