I worm in water so have no idea about putting meds in feed. Why would hard water make a difference? We have it here and in 8 years of worming I've never had a problem. Your question about worming during molting is interesting. You might post the question direct to "Resolution" that is active at posting on backyardchickens. If you get an answer could you please share it. Back to worming.....I do pour the water with wormer in it back and forth from their water bucket to another bucket to keep it mixed well at least a couple times a day.
Some minerals in hard water might interact with certain medications.
-Kathy