For sand bed I wouldn't suggest anything deeper than 4 inches... the problem is that cat sharks like to dig and rummage in the sand bed looking for food. Because of the denitrification process being what it is, how it functions, etc. that shark would be releasing all of those gas pockets into the main tank, which will create for some heavily toxic conditions for that animal and any other in the tank.
When it comes to other fish with the cat shark, that may be tricky. The cat sharks, by nature, are predatory and have a mouth full of very sharp teeth (capable of breaking through shell fish). They also are a very shy animal. When it comes to providing the proper environment for the shark that is going to leave very little shelter for any species of fish that goes into the tank... which is likely to mean the fish becomes food at some point. Let me give this some thought. If I can come up with a species of fish that would work in that size tank with a cat shark I will surely post about it.
As for filtration, I also would not go with something like in that link you posted, Pasfur. Sorry, but after keeping a number of species of cat sharks over the years I don't feel that would be sufficient or practical. I have asked my husband to help me write up a post about the type of filtration needed for something such as this, but neither of us has time to sit and type it all out right now. I will be back later tonight or tomorrow sometime to do that. I am going to strongly advise the use of a 75 gallon refugium on this system if you're going to consider the cat shark.
I'll be back later with filtration details for you. If its ok, I'd also like to pm you about where to find a marbled or bali cat shark without too much difficulty, and its a reputable place to get them from... if you can find a way to do it. The place only sells to wholesalers, so I would have to give you some specific names and contact info/ideas in order for you to obtain one. (thats why I can't post it here in the open thread)