I would try it. My five are great and they got along with Karen when I added her... of course she wasnt in there long before I realized she was sick.
Just give it a try. Do what I do, have an emergency bowl and net nearby and just watch them closely. Like DQ said, they will re-evaluate each other but after they figure out who's boss it'll settle back down. I do think 6 is the limit for a 10 gallon though, just my opinion. I'll be adding Rosalie back in once she is healed to complete mine.