There is nothing in the original post to suggest parasites to me. I think the issue is different.
Almost all disease and health problems in a fish tank are due to stress. And there is a lot of stress going on in this tank. A 15g is not sufficient space for all these fish. Some of them need a group (the tetra) or they will be stressed and likely to be aggressive. Even if they do not physically attack other fish, they are sending out allomones and the other fish are reading them.
Ammonia at any level above zero will stress fish. And molly are especially sensitive to ammonia, so that likely weakened them.
No mention is made of the hardness (GH) and pH. If these are lower than what the livebearers require, that adds further stress as it affects the fish's physiology. And the higher temperature played into this, as fish have to work much harder the higher the temperature. More stress, and more weakness.
Lower the temperature to around 77F. Do a partial water change of maybe 1/3 the volume. Do another tomorrow. Go from there.
At the same time, take a look at your stocking, as there are problems as noted above. And there is absolutely no room for more fish without removing some of what is there. We have fish profiles, second tab from the left in the blue bar across the top of the page, and these fish are all there. Data on minimum numbers, tank sizes, compatibility issues, etc. are included.