Ringworm is a skin fungus (a cousin to athelete's foot) and is so named because it frequently makes a circular ring patch of missing hair. A cat (or human) gets ringworm similar to athelete's foot in humid moist conditions. We frequently see more ringworm cases at work during very rainy or humid weather. It can be contagious. The only way to know for sure it's ringworm is to have your vet look at it with an ultraviolet light called a Wood's light and do a skin scraping.
If the new cat had a bald area and the other cats didn't have a problem before this then I would be suspect of something contagious.
Also food allergies can take 2-3 months for the allergic reaction to go away even after the food has been eliminated. Your vet should be able to give them something for the itching relief in the meantime.
Revolution isn't going to help with ringworm or food allergies.
Your money might best be spent with a vet visit to determine exactly what is going on and the best method of treating it rather than spending money treating the wrong thing and the situation gets worse requireing more expensive treatements later on because of waiting.
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