I currently house a 1:7 in a 20 gallon long. It was a 1:5, but with the introduction of a new replacement male in one colony (wild snake ate mine) he did not get along with the adult females and killed several newborns.
So I gave him 4 6-week old females and broke up the colony of adult females between my other two colonies. I also separated the babies into 2 categories, pinks and fuzzies and above. One colony of adults got the pinks, the other the older babies. They don't seem to mind so long as they have something.
So I've now got:
a 1:4 in a regular 20-gal
a 1:7 in a 20-gal long
and another 1:7 in a 30 gallon thats about 6 inches longer than the 20-long
I'm hoping to get all of my colonies set up in 20-gal longs. They're easier to clean as they're not as high on the sides. And when you're short to begin with and they're on a table, reaching over to scoop out is a pain. =P
I think so long as they're not stepping or tripping over one another they should be ok. I've noticed mine seem to have a lot better emotional health when there's more in the tank. They don't like being in this big empty void alone, or nearly alone, so the more tankmates they have the healthier they seem all around. I guess I can't say that I blame them much.
Just remember, the more you have per tank, the more frequently you have to clean due to smell and waste. Their urine contains quite a bit of ammonia, and in a closed off container without good ventilation that ammonia buildup can quickly spell death for mice.
Hope that helps a little.