i am also looking at the ferret nation 142. it will cost me 161+change to get it. i will be putting 2 girls in one half and my little boy in the other do you thing that this cage will be too big?? i mean my little boy is only 2 months, he is still tiny.
If you get the add-on to add to the top, you'll have to cover the hole in the bottom of it with plexiglass. You certainly don't want any mating through the bars...
Ivratz is right. You should definitely add hardware cloth or wire mesh to the outside, as the spacing between the bars is quite large. This can cost a lot, but it's well worth it because you don't want lost rats!
In my opinion, a cage can't be too big. They may seem a bit intimidated by it at first, but they'll worm up to it in a little while and then they'll love it.
Keep in mind, although your boy may be tiny right now, he'll almost certainly grow!