Depending on what you're feeding, it's my understanding white stringy poop isn't necessarily a cause for concern in a clown. For instance, if you're feeding mysis primarily. If they're not looking bloated, nor skinny, and are eating, I'd not rush out to treatments.
Also, I don't think general cure is reef safe. If you're soaking food in it, and then putting that food in your tank, it's the same as adding small amounts of it directly to the tank. Focus mitigates that somewhat, in that it's not initially floating free, but anything that doesn't get eaten will release it at it breaks down.
I'd run some carbon to reduce the chances of your clean up crew or corals getting nuked by the treatment. Well, what I'd really do is avoid doing this in the display tank, but that's probably obviously the recommendation.
Also keep in mind every additional fish and coral you add is one more chance you're going to infect your entire tank. So if you're going straight into the display, I'd make sure you're very confident in your trust of the source of things and your ability to notice the signs of issues when watching them before purchasing.