You can add the Stress Coat you want, but if the fish are nipping each other, then you have the potential for finrot.
For what it's worth, black skirts (by which I assume you mean Gymnocorymbus ternetzi) are nippy, but less so when kept in big groups, and may even be viable community fish in the right tanks.
Serpae tetras (Hyphessobrycon spp.) on the other hand are notorious fin-biters, and do so in the wild as part of how they feed. They also have a true "feeding frenzy" that makes them a liability around slow-moving fish (guppies, angels, etc.). I wouldn't keep them in anything other than a single species aquarium. Some years back in a TFH article I put these fish close to the top of my "ten worst fish" list. May even have been at the top; can't remember.
As for red-eyes, assuming you mean the South America species Moenkhausia sanctaefilomenae rather than the African one, well, these are fairly good fish but can't be kept with slow-moving tankmates. They do need to be kept in big groups though, otherwise they become more nippy than otherwise.
Since mollies more often than not need brackish water, and certainly will be nipped by nippy fish, I'd scotch that idea right now. Concentrate on upping the numbers of tetras you have, and perhaps rehome the serpaes if you can. They look great in Amano type tanks where you can have 20 or more schooling about, biting each other if they want, but otherwise doing no harm.
Cheers, Neale