You will be making a stock solution, not using the crystals right out of the bag.
To make the stock solution, add one-fourth cup ACV and one-half cup copper sulfate crystals to one quart of water. Mix until all crystals are dissolved.
Then from this stock solution, add one and one-half...
No, it won't harm the others. But it's expensive to treat a whole flock when just one has the issue.
Also, while Medistatin is recommended to treat gleet, there are alternatives that are a fraction of the cost and work as well, if not better. Copper sulfate in the water can treat it, and Epsom...