I just ordered them a martin r-685 which I think they will enjoy. I'm gonna equip it with lots of goodies. hammocks,tunnels etc. I might even add one more rat to the group haven't fully decided tho. Don't know how hard an intro would be and if it goes sour I have no where to put the new one.
Edit: Currently they are in a 10gal with that attachment thing which has two ramps for them to go up. It is fine for them until I get the martin tho.