I wish I knew how to help you, but unfortunately I have no experience with corals. The most I can do at this point is give you a care sheet link. I also did some research on the web about them, and there is conflicting information. Some sites say low flow and others say medium to high. I would try each of the flow areas (low, medium, and high) to see which it prefers. If flow is too high for its liking it won't open (that goes for most filter-feeders). Here's this link for the duncan coral care sheet (http://www.asira.org/duncans)
I wouldn't believe everything the pet stores tell you. I've heard many false statements from pet stores both in person and on here. I would say the chances of parasites are the same no matter how many corals you have or fish for that matter. Unless one of your organisms bring in a parasite, the chances of increased parasites, I think, is unlikely.
As for your refugium, I have a link for you below. Hope this helps.
Refugium setup


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Phishie
Site Coordinator

"An ounce of prevention is worth a pound of cure."

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