In the past I've quarantined new-from-the-LFS fish for 2-3 weeks before adding to the main tank with Clowns, just to be on the safe side with my main concern being white spot. How big is your existing tank? If you can get good healthy clowns from an LFS you trust then I'd be inclined to add them straight to the tank to avoid any extra stress, unless your tank is huge to the point where treating a white spot outbreak is going to be a real hassle.
I always keep my quarantine tanks pretty simple; bare-bottom, no/minimal lighting, and just enough decor to keep the fish comfortable. But as you can see from my thread that someone else linked to I've had a couple of bad experiences with setting the tank up recently, so it's not without potential problems. If you do decide to quarantine then make sure your tank is well established before adding the clowns.
