The smaller the group, the more squabbling and the more in a group justs make the fish feel more comfortable and you'll get a lot less troubles as they act more natural and you shouldn't have any hunger strikes etc. I've kept 5 in a tank before with no troubles at all but it's all down to the individual fish itself, you can get some right nutters that never seem to fit in however many there is in a group! You don't have to have 5 anyway, you still have room for 6, just keep an eye on the water (but that should be happening if you have 1 Discus or 6 anyway!)