nitrAtes are at 0.0? Planted tank? What do you use to test?
As for the guppy's tailfin, is he a fancy, or does he have an unusually long tail for a normal guppy?
In your 10G tank, what is the stock?
As for the fin rot -- Many long-finned fish develop fin rot as a secondary infection initiated from nipped fins. In a 10G tank with only 1 guppy, 1 platy and 1 molly, this would be nearly stocked, and any more could easily casue some aggression from over-crowding IME.
In my own experience, I have never seen aggression in the Long-Fin Skirt Tetras that I have, but he chronically has redness around the ends of his fins. I have not seen aggression per se, but on occassion I do see a tiny 'piece' removed from time to time.
Like you, I have tried treatments, but to be honest nothing works like really clean water. In my 30G I do heavy gravel vacs every 5-7 days with 50-60% WCs. When I get lazy on this routine the redness returns. If I keep it up, he never shows any signs of fin issues.
Just my experience, but I would say that in a 10G (and assuming from your Sig) that you ahve all 3 species in this tank (and more than 1 each) that you are seeing aggression due to the confined space, to say nothing that Mollies in particular tend to be quite aggressive towards their smaller cousins (the Guppies) I wouldn't be suprised that you are seeing Fin Rot as a sign of a secondary infection from minor nipping and poor water conditions b/c with the tank and fish stock you would really need to do heavy WCs IMO to keep up w/ the heavy bioloads.