Hi Scott,
Okay, assuming you have a mature fighting conch and that 2/3 of your 30g refuge tank sand equates to roughly one square foot of surface area...then yes, there's not much food for it to go on and, without offering food, it would slowly starve.
Assuming you have already given this consideration but, aside from a desire to offer it a proper substrate, does your refugium benefit from it being in there?
As for the main system, if the gravel is too course, then is a slow conversion to a sandy medium an option for you? If so, that's what I'd do.
Not sure I've helped with your dilemna...but always good to bounce off ideas.
Cheers,
Bob