You only feed the medicated live food 1 day each week, both morning and night feeding. Then you feed frozen food the other days. Yes, I would think that they would go back on live food again if you feed it to them for nine weeks straight. I know I've given mine live mysis as an occasional treat and they loved them, but didn't stop eating frozen.
Of course, most of the time when you deworm, it's wild caught horses that haven't trained to frozen yet. Also tank raised horses should be dewormed, but I'm not sure if those come trained to frozen or not. I've bought all my guys from breeder so (knock on wood) I haven't had to go through the deworming process myself. I've thought about pipefish, so I've keep the protocol just in case.
Lois, ghost shrimp are fresh water. If you have a LFS that sells freshwater fish close you may be able to find them there. I think I've seen them at walmart from time to time. Where did you get your horses? Is there a chance they have internal parasites or could the stringy poo just be from not eating?