I wouldn't recommend keeping more than one species of cichlid in a 30 if you plan on having a pair. A couple of singles (of different species) might be alright, but I'd still go with something other than a GBR. The black tetras in particular wouldn't appreciate the warmer water, they're actually fine at room temp. Not sure about the Cory cats, some species are more tolerant of warmer water than others, best to check on them.
If you want a pair of keyholes it's best to start with 5-6 little ones, grow them out together, then swap off the extras after they pair up. Keeping 2 males together in a relatively small tank will result in one being stressed by the dominant male.
Cichlids tend to argue more with others (particularly rival males) of the same species.