My two heckeliis seem fairly mellow, they're about 4-5" now and get puched around my a 3.5" surinamensis. I'm sure they're both males and they seem to get along ok, with the occasional little peck from the larger of the two. They're beautiful fish, and you should definately get some. Sytart with a group and thin them out if/when needed.
very nice fish i have one in a community tank with 3 other geos and a bunch of ca / sa fish mine is about 4-5 inches right now. he eats from the surface and bottom great fish