Very similar to H.spinifer which can get up to 6+ inches long. The 3 I have(H.spinifer) are about 5" long...I think the last molt they had was to their adult size though I suppose they might have one more molt, though I doubt it.
Yes thay can be kept communally. I keep 3 adult H.spinifer in a 20 long tank though I could probably keep 4-5 in there safely. It's best to have them all about the same size. There is a pic of my setup in the Enclosure thread in this forum somewhere.