Hiya Souffle,
I think the Gabber would be a great size for a pair or a group of 4 - i would try to avoid keeping odd numbers of robos, as this sometimes ends up with one robo being left out, or one robo being bullied. With even numbered groups over 2 you will usually find that the robos will pair off, but this isnt a problem as there will not be any robos left out.
With the Gabber you would need to make sure there is nothing for them to fight over, so would need to make sure the food is scattered (not in bowls) and the water bowl or bottle is easily accessable to all robos. You would probably need one wheel for each robo in th cage, but you will probably notice they all want to use the same one, lol.
Some people do find that the bigger cages can create mroe problems due to the fact that there is more space for each one to create its own territory, but i have never found this, but i do try to keep my cages as simple as possible to avoid squabbles.
A rody sized cage would be suitable for a pair, but i dont think it would be suitable for any more than two - although if you tried it you could see how they got on - maybe it would stop them getting individual territories? I am not sure - maybe see what aother people's experiences have been

Hope this helps - please just let me know if you need any more help

Tammy x