RG's can be kept in groups and will form haraems the same as Orchid dottybacks. Problem with the grammas is that the change sex VERY quickly (something daft like in a week!!). I think female to male, but not quite sure which way around. though it is only one way.
here in the U.K all RG's in stores are kept individually and are usually near enough fully grown. So all we can be is probebly male fish, that will not get on with each other.
If a store has more than one gramma in a tank they should be fine together when you take them home. Or i would be tempted to try very small fish if they are available to you.
HTH
Mark