It would be highly unlikely they would get stunted at those sizes. You could most likely keep them in there untill 8-10", where the width would start to become an issue, without having stunted fish.
Personally i would find it a bit too crowded, but there seems to be no scientific answer as to why you couldn't do this. However, chances are they will start killing each other off as they get more mature. A longer footprint and some good decor would help prevent this