Just a few things:
running filters don't cycle a tank, so adding an additional filter, as someone already said, won't add anything to your tank in terms of establishing beneficial bacteria.
with your extremely high bioload, the ammonia and nitrite will skyrocket quickly and you will not be able to manage it with water changes alone IMHO.
A few suggestions to avoid a lot of dead fish:
get used i.e. cycled filter material from a CCA member or maybe a helpful local fish store (not Petsmart)
buy several bottles of Tetra Safe Start, shake well and add all the bottles, then keep several bottles on hand to use as necessary
remove some of the fish, if you can, and put them in another tank termporarily to reduce the bio load
Feed sparingly or even not at all for a few days
A few other things -- yes, adding fish to an existing peacock/hap or mbuna tank can be difficult. There are few ways to manage this, including:
always add several at a time (not just one or two)
rearrange the tank to break up old territories
add immature fish -- not babies and not mature fish (especially males)
Pick your species carefully
Which also brings me to a question about your stocking. is this an all male peacock/hap tank? This is a difficult tank to manage and you need to monitor it closely for aggression. You must have a second tank set up to pull bullies and/or to pull those who will inevitably be bullied. I've done such a tank, so I speak from experience. It's a delicate dance to get the right balance and you WILL need to pull fish.
You should also be checking for nitrAte. Only then will there be evidence that you've got beneficial bacteria established.
Good luck!