My bristlenoses regulary fight when in close proximity to achother, usually 3-4" or less. They usually erect thier side stabbers and try to swing at them, as if to literally stab them. The worst that has ever come from this is a little scratch and severe fin splits. Usually the standard fight has fin splits, but they heal within days.
As most plecs get bigger (8"+), they should be feed a more high protien diet. Many commons, sailfins, gibbi's, panaques, adonis, titanic etc all all meaty foods when large.