it would be a risk because i have heard that if they pair up,they would kill everything else in the tank. u could try it though.
Nothing more than a false rumour I'm afraid, I've never had this happen.
It's true it may get a bit tense at first, but I've never had issues with this. If they pair they won't "kill everything else in the tank", just make sure you don't buy odd numbers, as a pair will continually bully the one left over.
In that tank, I see no issues, just keep an eye on your existing one to make sure it doesnt pick on new ones at first.