That sounds like a perfect set up for your 7 birds. Good job waiting between medications. They can take weeks to completely pass through their system. I was always told to wait at least 7 days before eating the eggs collected after dosing a chicken. Not sure if that really is necessary but I bet in time they will shape back up and get back to it. I wouldn't be concerned at all about your chickens and your horses. There are no diseases that pass from horse to chickens as far as I know of, no worms can be transferred from one to the other. I've heard that if chickens roam where horses eat they might be able to get salmonella from possibly eating droppings but I'm really not sure. I know our chickens love to tear through the horse manure to eat the fat maggots that live there. I bet your chickens love there home it sounds perfect and protected. I have almost no predators where I am except night beasts (racoons, skunks, and mice) so we lock up tight at bedtime!