I'm afraid I retyped my last post just before you replied
Going through your post -
The platies and gouramis were exposed to only very low levels of ammonia while the guppies would have been exposed to higher levels. I think that this, together with their weakness nowadays, is why the guppies are affected. Depending just how hard your water is, the gouramis may also be weakened. The top of the gouramis' hardness range is 268 ppm or 15 dH. Fish should ideally be kept in the middle of their ranges, but unless your water is very very hard, it should be less than 268 ppm/15 dH.
The instructions say that the floss is part of the cartridge which is a bag filled with carbon. I had not found the instructions when I first typed my last post, and after I found the the instructions I retyped it.
At first I thought the floss was a separate thing, but the instructions say it is part of the cartridge. This type of floss does not clog nearly as fast as sheets of floss. With sheets of floss the water flow out of the filter drops quickly but if your water flow isn't dropping, that's fine.
They tell you to change the cartridge every 4 weeks because carbon gets full. But you don't need to use carbon full time. You can either leave the cartridge there until it falls apart or cut a slit in the bag, remove all the carbon and stuff the bag with filter sponge, any make cut up to get it in.
The risk is that the carbon inside the bag will remove any medication you might add which is why medication instructions always say to take out any carbon media. Even when it's been in the tank a long time and is full, there is a theoretical possibility that a medication is more strongly taken up by carbon that what's already on it so the other stuff will come off and the med stick to the carbon instead.
Do you get much brown goo on the media between washes? If you do, that brown goo is a "nitrate factory" so you need to wash it off more often - every couple of weeks.