Hey Zoe,
I was keeping my 1.2 rats in a 33 gallon tank and they were tripping over one another. Put a food dish, water bottle, exercise wheel, igloo hut and voila ..one very full tank..
I gave up after all tank items kept getting moved around, knocked over.
I went out and picked up a small animal cage. , approx 4'x2' by 2 feet high, sell for around 100.00. Now the little family has room to move and they seem less stressed. Of course, they still tear everything up and move everything around..
But at least they have the space, so IMHO a 15 gallon is probably a little small. Be ok for the babies once they are on the "move".
Cheers
Jim