On my 72gallon planted bowfront I run 1 magnum 350, which is similar to your HOT magnums. In the container that is suposed to hold the carbon, I have filled it with Eheim brand "Ehfisubstrat" which is very good for culturing bacteria colonies. The blue sleeve outside of it, is mainly for trapping particles and waste and should be rinsed or changed monthly. There are other brands that you could use also, basicly you are looking for a porous ceramic or sintered glass material. Don't use any other media in the Hot magnums and when you clean the media just rinse with aquarium water. This will give you the most out of your HOT magnums.
With the whispers continue to use these and the carbon here(this will be more than enough carbon for your tanks). Do your whispers have a spong type material in front of the filter bags? This is your biological media, if yours are older you can stuff the front part of the whisper filter with plastic pot scrubbies this will give you more area to grow bacteria and make your whispers more efficent. Also when cleaning the pot scrubbies just rinse with aquarium water.
Remember chlorine will kill the benefical bacteria. Once you get your tanks back to the cycled stage and have some good bacteria colonies going, You could use your python to refill the tanks, just unplug the filters first that way no untreated water goes through them. Still there is no way to protect your fish from the untreated water though. If you do go with this method I would keep the changes to 25% and add the Prime(water conditioner as soon as you can).
By the way if you can keep the the ammonia down to .25ppm your fish will be fine.
