Since you are already aware of the problems in mixing the front and the peacock with the mbunas, I will answer your question without going into that,
77 inches of mbunas in a 55 gallon tank is just fine. That is about the amount of cichlids I have in my 58. Now, your problem is that you do not have the filtration for this amount of fish. Putting 77" of mbunas in there is great for controlling aggression assuming you have the right mix of mbunas, but you have to overfilter to compensate for the added load. In a normal tank, you want to generally combine filters such that they are rated at 10x the volume of your tank. The reason you have to have such a large mutliplier is because the filters are rated with no media and at zero lift. If you have filters rated at 10x your tank volume, you will probably end up filtering your tank's entire volume of water 2-3 times per hour.
Now, this is normal so for a heavily stocked cichlid tank, you need to go above this. For instance, in my 58 gallon mbuna tank I have a fluval 305, an Emperor 400, and a Penguin 200 running on it (I am going to replace both the Emperor and the Penguin with another canister here in a bit). Therefore, my multiplier is closer to 15.5x.
I believe your Whisper 60 moves around 330 gallons of water while your penguin moves 200 gph - giving you a total of 530 gph. Therefore, you are just under the 10x needed for a "normally" stocked tank (you are actually probably slightly over the 10x multiplier because your 55 is not really holding 55 gallons of water - but we will ignore this for the sake of this discussion - the point of needing to increase filtration as they grow still holds). You should be fine with this for now as I think I recall that your cichlids are fairly small at the moment, but as they grow, you will definitely have to add filtration. By the time they are full grown, you want to be up in the 15x range.
So to sum it up - 77" of mbunas is just fine assuming you up your filtration as they grow.