It won't hurt the other cats but you will not know how much your desired cat is getting. I prefer to add it to canned food. I usually split the dose and give 10 drops of DIA-IONX with breakfast and again at supper until there is improvement and then you can switch to a whole dose once daily.
In addition all of your cats would benefit from PROBIOTICS and OMEGA 3,6,9. If your dermatitis cat cat has not had a stool sample checked recently it would be wise to have one done again. Parasites negatively affect skin.
Please let us know how you do.