For a pair of coral cats, 300 should be enough. I personally recommend shying away from mixing elasmobranches and teleosts. You are asking for problems, especially in a small setup.
As for water quality - I highly recommend a large and efficient skimmer unless you plan on siphoning feces and doing very frequent water changes. The cats are pretty efficient at eating all of the food, but produce a lot of waste - having a primitive GI tract. They vomit indigestibles, have a short intestine and lack a colon.
You can see in the tank I'm setting up, that I have taken several measures to remove waste. The current skimmer is only rated up to 1000 gal and the tank is 2800 gals. I have a RK2 25PE that I plan to modify or may add a second cone.
The protein skimmer also acts as a degassing chamber.
Also, liverock can only be considered decor in the shark system. The filtration is inadequate. The bead filters are great choice. Great at biofiltration, water clarification, and making water changes. The amount of sludge they pull out is truly amazing. For your size aquarium the Ultima II 1000 is a great choice. I feed ~ 3 lbs of food at a time and though I run two beads on my current 900 system - there is never a spike in ammonia. There is also no need for a sump and you can plumb the skimmer after the bead and avoid a second feed pump.
