I'm talking about the length of the pond, not the width/height. From what I understand about sharks, surface area is far more important than height most of the time. (the 12 feet from the calculation vs the 10 feet you provided). I have no issues with how tall the pond's going to be (aside from being concerned with how you're going to support that much water). Although this is assuming the cat sharks are walking benthic (as opposed to inactive benthic)....
Regarding doing water changes, I've heard that you can buy some salt brands (probably IO) at wholesale price (as of 2015, I think it was 25 USD/box).
I'm also linking the EHM (and its 2007 followup) here as well. Still working on finding those sources.