In all the information I've read on them, not once I have seen them suggest they like to be kept in groups. That's just not common behavior for a carnivorous fish. Got a link?
Wile yes they do get long, their body is very flexible. A 29 would be the minimum I would keep one in.
About them getting out..Well both of the ones I have had got out. Each time it was through the hole cut out for the HOB. Now I had these holes cut as small as possible. This kinda leads me to think that if you have a HOB filter they can get out. Ideally you need a canister filter because you can cut the holes small enough that the only thing that will fit through them is the hoses for the filter.