Other species of halfbeaks for a start -- the males of bigger species harass/kill the males of smaller species, and big female halfbeaks will simply eat males of very small halfbeak species.
Beyond that, they mix well with standard community fish that stay the middle and lower levels of the aquarium. Avoid anything nippy or aggressive, but I've mixed them even with certain pufferfish and not had problems. Do of course consider water chemistry, as different halfbeak species prefer different sets of conditions.