Stringy poop is indeed a sign of intestinal worms however it is more commonly seen in fish that are not eating well. But I believe yours are WC aren't they? So that does increase the risk of worms. PraziPro would be what I would and have used. You would use the standard instructions on the side of the product. I have the 1oz bottle and wouldn't be able to read it if I wanted to. Where your horses didn't have stringy poop in WT and now does... I'm thinking it's an eating issue not necessarily a worm issue.
Everyone make sure you do not medicate in a diaplay tank!

One way worms are spread is when one animals eats another animals infected poop.... and horses can be poop eaters.... accidentally or otherwise.