Damsels might be a good choice. In that 29, no fish will have room to get out of your maroon's territory, so it better be able to hold its own. A blue devil or domino or something might just work out, though. Maybe go down to the LFS and get the meanest one in their display tank
You could have an interesting twist on the "aggressive" tank with nothing but small fish.
I think tank size is a big factor. Our 3" maroon was returned to the LFS by her original owner because she was terrorizing her 29. In our 100g there have been no problems so far.