
... even in a fully cycled tank, you do not want to add fish that fast. this is not a 'fully cycled' tank (although I had not realized that the filter media was that old).
Adding too many fish too fast increases the bioload in the tank too quickly for the bacteria to keep up. Generally, you want to add new fish very slowly so the bacteria cultures grow accordingly. As a result, ammonia builds up pretty quickly.
You'll want to be keeping up with those 20% daily water changes. Certainly do not slack on this or ammonia and nitrite levels will become very harmful. Just for reference, I do nearly a 50% water change weekly on my tank with 1 betta and 2 very small otos. Once the tank is fully cycled/adjusted, you can go down to weekly water changes. 50% would probably be fine at that point. I actually overkill water changes on my betta tank because it only takes 1 minute to almost fully change the water when i'm using my python.
pull all of the females out, redecorate, and re acclimate them to the tank. let the 2 new females loose in the tank first. Wait a minute or two, and release the others. You want to add the most aggressive female last.
make sure you have all females too! post pics for I.D. if you are unsure.