I have a hyper-aggressive (this is not an exaggeration) Hongi Island-his behavior is typical of his type but on the high end of Mbuna. He is in his own 10 gallon and has picky eating habits. I got an albina chocolate pleco knowing full well the possible issues.
The pleco outgrew the Hongi quickly, and only gets picked on for a day or two when the tank gets reaaranged. After that they each have their portion of the tank and do fine. The hongi likes to eat off the gravel and is very defensive of his food which can be an issue sometimes, but for the most part it works out. Some minor fin damage for the first few days but the pleco always heals up fine.
With that many Mbuna in the tank you might run into issues especially from the little guys who are expressing dominace. Be weary--get a Pleco that is matured, healthy and somewhat aggressive himself and you shoudl be slightly better off.
Either way you have to pay attention. It is a constant watch in my medium tank.
Another thing to remember is plecos are filthy, and Mbuna are just as bad. The two of them in a 10 gallon with 210 GPH of filtration manage to keep the water constantly filthy kicking stuff up, eating and pooping. My parameters are fine but I wouldn't say its an attractive tank compared to my main mbuna tank. With two and 9 mbuna I can imagine it being kind of bad.