Females can usually be housed together, unlike males. I have 7 living together usaully peacefully. I say usaully because sometimes there will be one with broken or torn fins. But it' s never seriously bad and it doesn't happen often. I have 2 very much older females living in there with 5 young ones.
If you already have a female and your thinking about adding another one to your tank there are precautions to take. First off your adding a new fish to the already established fish's tank. If you just release the new one, the fish that was already there might not like it to much and think of it as the other fish is invading it's territory. When you bring your new fish home, you have to acclimate it anyway, so here is some signs whether they will get along or not.
- Is the fish you already have getting vertical bars?
- Are they flaring at each other?
- Are they trying to bite at each other through the bag/container?
- Do they look like they are tensing their bodies, making a U shape if you were looking at them from above?
If they are doing some of these things then try letting your new fish stay in its container for a little while longer, so maybe the other one can get used to it being in it's space. Or when you let your new fish into the tank keep a close eye on them. They may fight, but as long as you make sure they're not hurting each other to bad they should be fine. Fighting is just a way for them to establish a pecking order. If they continue to fight, try adding one or two more females, that way there will be more of an order. When you have just 2 in there by themselves it's harder for them because they both want to be boss.