Hi all,New rat owner here, we brought home three handsome boys about a month ago, however, we discovered that two of our "boys" are actually girls!!! Now we have an additional 11 babies and pretty sure another litter on the way!We bought an 'All Living Things' rat starter kit. Pretty good size for 3 rats, a pain in the neck to clean, but, oh well. Once we realized we were about to have a whole lot more rats we bought another cage (this one a bit smaller, but all we could afford). Also got very lucky & found a bird cage & a med sized rat cage at a local thrift shop for $10 each. Great deals, but now our living room is full of cages & I'm constantly cleaning them!!We're really hoping to end up with a total of 6 rats - 3 girls & 3 boys. The ALT cage will do for one of the groups, but none of the others are big enough for the other group. I've been reading/researching cages & it seems the best there is, is the Critter Nation. I found the single unit CN on 1800PetSupplies.com for $112.49 which is the best price I've seen online. So here's what I'm thinking: I buy the single unit and keep the ALT on top of it till I can afford to buy the add on/second level. What I'm wondering is, do the units really fit together as well as if I had just purchased the double unit?Also how many rats do you think comfortably fit in a single unit? I've read reviews/comments from people saying they have anywhere from 2-5 rats in each level?Thanks!!