Feral cat colony with severe diarrhea
 

How would one approach this problem. Is there a general treatment I could try in stages to put in their food?

I don't know if your feral cats have the same problem, but when I got two of mine that were living in a colony, they had diarrhea so bad they no longer had the stregnth to clean theirselves, and were plastered in it and flies. They were diagnosed with Giardia, caused by a stream they were drinking from, and had to be put on Albon. Even if you could put it in their food, I doubt they would eat it, mine used to foam at the mouth because of the taste. Hopefully there is something easier for you to use, Good Luck.

Parasites most likely :(
How much can you get close to them, because some deworming medication can be applied on top of the paws and they'll lick it off the problem is being feral is not an easy task.