on the tank changing front. It is hard, but let the tank "mature". Filters and gravel are environmentally more helpful as time goes by - with routine gravel vaccuming and water changes. One year old tanks can be perpetual motion machines if maintained. :)
On the dropsy front: if those animals have only been in your care a couple of weeks, it is possible they were exposed to and already attacked by those things before you got them. Also, sometimes older animals are just more vulnerable to illnesses. That (sadly) even happens to elderly people where they get hurt one way and just all sorts of things go wrong.
This may sound a tad unkind, but balloon mollies, like fancy goldfish, are obviously arranged differently than the "models" designed by nature. There is always the question of whether their organs and immune systems will work as effectively as with the wild fish or - in the case of hobby livebearers which have undergone a lot of breeder selection - those fish a little more like the wild fish's design.