I got a new 7 month old kitten from the humane society, I have 2 other cats well the new cat came back?

cat came back pos. for coccidia, also had URI well treated all cats but had to stop per the vet on the other 2 cats, they got diarrhea so bad from the medication one had to be hydrated she is only 7 pounds and she was vomiting also, well it seemed that everybody was doing well but now the one who had to be hydrated started scratching and her hair is coming out around her neck and she has scabs, the new cat they did find some flea eggs but supposedly they treated her, I just put front line on both of my cats and also I noticed her eyes were watery and she did have a course of eye ointment also and her eyes looked puffy also, so I started to put more ointment that I had left over until I can talk to the vet, what is going on??? It's been a nightmare since I got this new cat and he is so cute and it has cost me over 500 dollars so far treating them.