Sorry if this reply comes a little late, but hingebacks don't do well in groups. Males fight endlessly, regardless of season, and females do much the same. Hingebacks are not especially active tortoises, either (compared to Sulcatas or Leopard torts). They are most active in morning and at dusk. Despite this, I still recommend as large an accomodation as possible.
As far as housing goes, I generally recommend kiddy pools. They hold water and moisture very well, which is important for these animals. Make sure there's lots of cover, etc.
Chelonia.org is a good place to start with caresheets and whatnot.
T.G.