Natural Ways to Care for Feral and Outdoor Cats

Hello, I feed a colony of about 25 feral cats. I have a wonderful community that supports me with donations. I am always looking for natural treatments to help improve the life of these cats. I have read a lot about DE as a dewormer and natural flea and tick treatment. I would like to add it to their wet food I give them every other day. When I feed them, there's no such thing as individual dishes of food. When I took over feeding them, I just put down a 22 lb bag of dry food and cut the whole front of the bag away. They can basically come eat when they want. When I take wet food out, I open 20 cans of wet food into a tub, then I spoon this onto the dry food for them. I usually spread their dry food between a few places at this time so they call all feed. If I added 1/2 a tsp for every serving of wet food and mix it in well, do you think it would be safe for these cats? They do live in a barn on a farm, although the owner would prefer they weren't there. I feed and water them and I do have an area where I have some cat beds with blankets for them. I know that straw is better than blankets but the owner doesn't want the straw in the area the cats sleep in. It's a tough situation, I can only do so much for them without the owner of the property objecting. So I look for small things I can do that will make a difference in their lives. DE sounds like it could improve their health, take care of fleas, and help their coats. Could anyone help me out with their thoughts on this?