[kyrilliondaemon]

For a group I'd go bigger than 70cm by 40cm, maybe 80cm by 50cm or bigger if possible. Unfortunately with young robos you also want 7mm bar spacing which makes it difficult...
The only ones I can think of that meet those are the zoozone 2 (once meshed), the Alexander (without shelves) or the mickey XL (also without the shelf).
I'd also give another wheel and water bottle if you can, plus check the houses have two entrances/exits and the hams can run around every item in the cage. Basically with those parts you want to make absolutely sure theres nowhere a ham can be trapped by a cagemate (or two) and attacked because that could lead to really nasty injuries.

Oh and silly I know but make sure you can tell them apart - it'll help determine if someone is being bullied or not and that you separate the right ham if separation is needed.