The gravel in my tank is 2-3mm, i have already seen him bury himself in it once so its obviously small enough.
Ideally yes, they would do better with sand, but you cant have everything can you? I'm sure that some of your fish are being kept in what can only be described as not perfect conditions hey Fishy411?..
And yes, they are very slow catfish, very docile but as i said very interesting.
Ben