IMO, you need at least 4 females to even out the aggression. I do not think that 4 would fit in a 6G, but someone may be able to say different. If you can't have 4 and you don't want to divide the tank, I would go with one. The females can be just as aggressive as the males. I bought 10 for my 20G and they still try to claim territory from eachother. They would need lots of plants or caves to hide in as well. I would also wait for someone else's opinon.