From your list, I doubt harlequin rasboras are the culprits, as they're peaceful shoalers. Neither the neons. Do not know about serpae tetras ... perhaps it's the platies or the gourami? Besides ... I know it's none of my business, but your tank is very overstocked, and overstocking may easily lead to aggression among fish, even if the fish are naturally peaceful. Plus, I'd personally recommend at least a 30 gallon tank for angelfish.
If you are not planning on getting a larger tank for all of your fish, at least provide enough hiding spots for all of them to disperse the aggression, and so that the angels have a place to hide when they're being harassed. Also, angelfish and gouramis are known not to be good tankmates.