I think they're a hit and miss with mixing. I had 2 pep shrimp, with 2 cleaner shrimp in my 9 gal before it crashed, and they wheren't fighting, but they also wheren't friendly. In that tank though, they had a lot more hiding spots, and I rarely saw the peps. I've yet to get peps for my 20 gal as I have no aptasia yet.