I would also not separate them. We needed to do that with ours because we had two boars fighting over one sow. When we got three more, we had no choice but to pair each boar up with a sow because the boars among themselves would always start fighting because females were nearby. So now we have three couples (the males have been neutered).
One big disadvantage of separating them was that, when Basil died last year, Spotty was by herself. We had to get a little boar from the animal shelter because we couldn't put her in the cages with one of our two couples. If you have a bigger herd, if one gionea-pig dies, the others won't end up being alone.
Aside from that, I have seen herds of 4 to 6 piggies and had the impression that it was very good for the piggies.
