i dont see how you would assume that (unless its because they are cheaper then most others) but yes it was infact a common. i guess that makes you sherlock holmes
actually, it makes perfect sense:
1) commons are about the only inexpensive "supersized" plecs that are readily available on the market, thus are the most likely candidate for a big plec to test in a predator tank.
2) commons are also known to be fairly aggressive in protecting their territory and are therefore more suited to life with semi-aggressive predators (like red bellies)
3) commons are also one of the sturdiest of plecs and are thus more likely to survive and fight off the inevitable exploration attacks of said semi-aggressive preds
so, no, not Sherlock Holmes. just one more member with a decent amount of experience; after a while, most of these things can be deduced.
--edit--
that being said, i have to agree that not all plecs were created equal. a lot of the fancier ones are very shy and retiring--although not all of them.
but common plecs and bristlenoses are good little toughies due to their territoriality. but even then, some cichlids simply will not tolerate any competition and you'll be better off just going without.
you do know that most mbuna are primarily herbivores, right? with a magnetic algae scraper, there's really no big reason to have a plec in a mbuna tank unless you just really, really want one.