The tank is perfectly fine for very short term care of a newly hatched shark. But it will out grow it within a year. It should also be noted that cat sharks primarily eat crustaceans and do not pose an immediate threat at all to the clowns. IMO, the clowns may be a greater nipping threat to the shark.
But I've raised around 20 or so of these sharks from hatching to about 18" and it will be fine for the short term - emphasis on the short term (under a year) - before it needs a larger (aim for 180) tank.
However, I do think you need to decide where you are planning to go with this tank. Reef, anemones or aggressive. You should start thinking about that very soon, IMO :yes: And I would be wary of the LFS as well.