How big is your tank, and how long has it been cycling? I find it strange that the guppies are the first to go.. usually they are the toughest and stick out any disease that would literally wipe out neons and gobies. Speaking of gobies, that bumblebee would really be more suited to a species tank. Usually it's specialists that keep them, not people interested in community aquariums. Oh and they are brackish water fish, meaning that they need some salt in the aquarium to thrive. The salt won't hurt your livebearers either, many of them thrive in brackish environments. Anyway, about the disease, try melafix. It will usually cure most minor injuries and early stages of bacterial infections.