guppie and mollie issues

[vic021]

Ok so recently i have had 1 guppie die along with 1 balloon mollie i noticed that the balloon mollie had a lump on one of its fins.So i am treating the tank with some external parisite medicine that treas ich velvet and other external parasites.Ever since the first balloon mollie died ive noticed my baby guppies nibbling the top of my other balloon mollies head.is this something i should worry about? what should i do? i currently have kilifish hatching and growing in a seperate tank how long should i wait before i put them into the main tank that im treating with medicine