Yeah I had one at 3 inches and in 5 months he grew to 8 inches. They can grow fast but they slow down I guess. Sizes of 20-24 inches isnt likely. Think more like 14/15/16 inches.
They are shy at 1st and reclusive. But will come out more when settled in, theyre nocturnal so dont expect to see it swimming all over the place all day. Mine hid 99% of the time and would come out only to eat when there was liight, other than that the lights had to be out for it to wonder around in the tank.
Theyre not really mean, but they are tough when they need to be. They mostly want a log or hole to hide in and be fed, but if fish try to steal their food or territory they will bite. Also, its not adviced to put them with fish that will harrass them constantly.