1 inch of fish per gallon of water is bunk. That's not even a working theory for FW, much less SW systems.
For a q-tank, you want to keep you load light, and preferably not mix fish that didn't come from the same source (otherwise, if they both have something nasty, both will get the other's ailment, as well as fighting their own. Not a good situation). You shouldn't put in a fish that can't comfortably turn around in the tank, and you should be certain to maintain pristine water conditions--large water changes is usually the easiest method. Some cheap silk plants can be added to provide some cover, without providing a place for the critter to get into that you can't get it out of.