Is it okay if i add med. to my main tank even though some fish are not sick?

my main tank is 30 gallons, it is heated,filtered,cycled and has 2 platies(male and female which is the sick one) and a male dwarf gourami along with 4 peppered cories and 2 ghost shrimp.i also got some snails in there and a hornwort plant. my female platy has a white spot right on top of its mouth(not on it) and it seems to be fungus. i have med. for that so is it okay to add it to the tank even though some fish are not sick. and will adding medicine to the tank kill the ghost shrimp,snails,or plants? if they will then what should i do? the only other tank i have is a 2.5 gallon tank,i can get some water from the 30g tank and put the med. in there along with the sick fish but when i put a sick fish in there they get stressed out and die even faster, i only got a heater for the smaller tank. so what should i do. and i was thinking of moving the 2 ghost shrimp into the 2.5 gallon tank and get a small filter for them and keep them in there because i had 3 ghost shrimp and one died because it wasnt able to find food, but whenever i drop in a bottom feeder tablet or shrimp pellets the cories always end up eating them and they dont share. or if they got a flake and they are eating it then the cories see that they got food so they scare them away and they drop it so if i put them in the small tank they are the only ones that will get the food so should i do this or is it too small.