We have a large two story enclosure, which is open on the verandah so they can all find their own spots. Plenty of spots for the piggies to hide out. But honestly I watch them closely and ocassionally Willoughby will get in Anouk (gp) road and she will squeak at him and he moves quick smart! She's the boss, they've never bitten each other and the guinea pigs are the same size as our smallest rabbit. They snuggle together, eat fine together etc.
I didnt realise she was pregnant till I felt the big lumps. Shes a bit of a pig when it comes to food so I thought she was just fat!