Full grown a Firemouth
will average about 6 - 7 inches, and they are much less agressive than things like your peacock and usually also the pikes. Pikes I notice tend to be per fish, and personalities vary greatly, but space makes a huge difference.
Also remember that a fish doesn't have to be large to be aggressive enough to fend for itself or to attack/eat a larger fish. I have seen a 1 - 2 inch Convict Cichlid
tear apart a 7 inch Oscar
in less than 24 hrs, and that in a 75 gallon tank.
As for the size of the fish you'll need to add, again it depends on how the tank is decorated and how the fish are introduced. If it's done all at the same time and the decorations are dense enough and plentiful enough, you'd be surprised what can work. A 7 or 8 inch fish isn't going to follow a 3 inch goby into a tiny cave with a 1 inch opening... especially if the cave is on the other end of a 200 or more gallon tank. If that goby has say 5 - 10 caves like that strategically placed throughout the tank, where it can get from one to another... with lots of dense plantwork (silk works great for this) anchored around the bottom, it can be done.
When trying to accomplish things like this its more a matter of knowing your fish and how best to shelter them from each other until they are all close enough in size to not be eaten, and providing them enough appropriate territory of their own for their needs.
Another concern I have is your water params considering you have African cichlid mixed in with Central American cichlids. You're talking about soft water and hard water fish all in the same habitat. Long term this isn't going to be good for someone. Central and South American cichlids can usually work well in a tank with floating live plants, which helps to eat some of the nutrient levels in the water and also provides wonderful shelter and territory up high, but African cichlids are primarily vegetarian, so they will usually eat and/or tear up most plants.
Another thing for you to consider is the diet differences in the 2 types of cichlids. A good standard pellet food for the peacock would be spectrum pellet food alongside of frozen forumla 2 food and algae wafers, but with your others things like Hikari cichlid pellets will be appropriate, along with live foods such as worms and brine shrimp, etc. (veggie vs high protein)
Have you considered seperating them?