[i have crabs]

yes cleaner shrimp would get eatin fairly quick in your tank, and garlic dosent cure ich at all it improves the immune system of the fish so that they are able to fight it off better,
in my opinion it would be best to buy a used 90g and use it for a month as a qt if the fish start looking real bad or stop eating, a few specks here and there isnt much to stress about though so as long as things are under control id just start soaking their food in garlic and hope for the best