If you plan on keeping those fish in the 55 then I wouldn't add any more. The labs reach ~5 inches or so each. They get pretty tough depending on what the other cichlids are.
You can add the tropheus but, no matter what you add in there you will need to move them into another tank later. 15-20 in a 55 is not an option IMO, unless you are using it as a grow out tank.
So have I. It works, but rarely. I have six in a 55 right now, 5 males and 1 female. They fight like dogs. When you get them young you have no idea what you have until they mature (like I did). If you buy adults then you pay a heck of a lot more and you don't get to watch them grow.
"patriotsfan" if you try it make sure you are ready to lose some to aggression and also be ready to spend money for caves and rocks for them to hide behind.