You have to start some where. When making recommendations, I tend to be conservative as a result of running what I thought of as responsible fish shops. I have been dealing with aquarium headaches since I was 18 and that was a long time ago. Stocking isn't a science, but there are some lessons to be learned. My dad use to say there was nothing to be learned from the second kick from a mule. If 80% of those you talk with have had disasters created by the same poor stocking decisions, you tend to lean in that direction. There are always folks that have had a different experience, which you fairly note, but 80% means there is a good chance you won't win the gamble.
Back to the cat shark, when they are little they eat a lot of food because they grow fast, which is where the filtration comes in. I would guess feeding them properly is a big part of successfully raising one. We have a local LFS
that has a shark tank, a pool that's about 1000 gallons. They seem a bit cramped to me, but they are healthy.