Gabbers are perfect and you do not need a nest box either in them as Holly said the shelf if a well ventilated cozy spot. None of ours have ever had any problems on the shelf either. The only alteration we made is to drill a low hole for a water bottle as the babies are too small to reach the one in the lid and they do drink quite early on from it. I think you will find the sexing OK as by 4wks you can usually tell and if you cant they are unlikely to be old enough to breed anyway! If in doubt put it in the male tank. Better one pregnant female in with the males than six pregnant girls who had a boy in with them!!!
There are pics of adults in the articles section but babies are a bit more tricky. We can usually spot the nipples on females through the fur.