I know that Matt keeps them in small tanks, but IMO and IME they need some swimming room. Mine are in a 4' x 1' x 1', which works pretty well for them but they use every inch of the 4', all day long. Now they aren't strong swimmers like tangs, but they do use the space.
If I wanted to put a pair into an established tank, I would be absolutely positive that I had another tank for food training, _completely_ cycled, with a large cleanup crew, and already accustomed to a large bioload (lots of food). Ammonia is hard on these fish. For that matter, you should start getting your cube accustomed to four meals a day because you _will_ be feeding that often to keep these fish. That's one of the reasons that I don't really think these fish are good candidates for SPS tanks, unless you feed the heck out of your SPS. (I do. But I'm kind of crazy.) But I still don't have mine in a coral tank.
I would also plan for doing at least one water change per day for the foreseeable future if they are in a smaller water volume. You will be dumping food into the tank. But, again, they need good water quality.
Do you have a large frag tank set up and growing well? If they never take prepared food, then you must give them a steady supply of corals. Where are those going to come from?
Do you have the time? You _have_ to watch them eat (not just dump and leave) at least four times per day until they are fat. Then you still have to feed them four times per day, but you don't have to spend as much time because you have a greater cushion if something goes wrong. It took me three months or so to get them to that point. EDIT: And, in the beginning, if I only managed three meals in a day with them, I could see them lose weight.
One thing that seemed to help them eat for me (and my experience was a little different than most others) was to have a small, non-threatening fish with them that _is_ a strong eater. It kind of seemed to help them decide what was food. But, they are _slow_ eaters at first, so that means even _more_ food so the files get their share. I started them with an absolutely tiny convict tang, believe it or not. That sucker grew _fast._

Juvenile cardinalfish seem to work well, too.
Sorry. I know these are tough questions, but the fish deserve you thinking about these issues before you buy them.