Would not add four or five females to a 10 gal tank unless plans are in place for the load's of fry the fish will produce about every three to four week's for each female.(hundred's in no time)
WOULD provide some live or artificial floating plant's so the there are area's for the fish to escape/ rest.
Would NOT add salt, but Would perorm weekly 50% water change from now on .
Male guppies will chase and nip at each other but with five or six in 10 gal ,with the afore mentioned floating cover,,regular water changes,, the males should be fine assuming water parameter's suit them.
Fin rot, fungus,are more symptomatic of water quality issues.
Healthy fishes seldom get either fin rot,or fungus,or many of the other parasitic pathogen's.