Its not the ammonia spike that will kill your inverts, if the medicine says it will affect, it will massacre ALL inverts regarless of shrimp or snails. On what chemicals you could use, you'll have to wait for someone that has gotten fungus in their aquarium.
20G heavily planted tank with Eco Complete
Pressurized CO2, Eheim Ecco 2236, 1 55W and 1 36W bulb
I understand what your using the medicine for. I was saying even if you remove your shrimp I would be hesitant about using the medicine, since the massive amounts of snails dying would cause an ammonia spike. Plus I was confused when you said fungus treatment, if its blue-green algae your after, than its a bacteria you are looking at.
I know that maracyin kills it also. i just got done treating my 55 with it and it worked great. I used half dose, which was 2 tablets in my 55. He said use like 5 for it, one time. Should see dead stuff in 24 hours and al should be dead in 48 hours. i have discus and tetras in there, that why i cut the dose and they are all fine.