An 8x3 rectangle would be fine for either of the true catshark species. Just make sure the perimeter of the tank along the bottom is completely open. In other words, don't stack rocks along the back wall of the tank. Catsharks are pretty active for a benthic species, they may not show it during the day but at night they're all over the place.
An 8x3 would be fine for a pair, if you want to go that route.
Tangs usually make good tankmates, just keep them well fed. They've been known to graze on the eyes of benthics from time to time - generally a pretty safe bet though. Much safer than an angel, trigger, or puffer.
Get some pictures of the shark first. Google pictures of Atelomycterus marmoratus and Atelomycterus macleayi. Those would do fine in an 8x3. Marmoratus is in the top 5 most common sharks in the hobby. Macleayi doesn't show up too often, although it's possible they show up labeled as Marmoratus from time to time.
For whatever reason, distributors can't label sharks for $hit. Brown banded bamboos, white spotted bamboos, coral cats, and marbled cats are all lumped into one name that's usually something like "Brown spotted marbled bamboo catshark". It's a mess.
Keep us updated. What's the filtration plan on this tank? 8x3 would be in the 360-450 range, depending on the height you go with.
Drew
