I'd say try a small chain-link ( if you can find one ). I have one that is about 10 inches long and it is great. I have no problems with him eating my othe fish or bothering corals. He crabs a hermit that is holding food , once in a while. But that is to be expected.
About being active, the chain link is out all the time. He swims around the tank and comes to the glass whenever I'm around. I do hand feed mine, which is probably why he is so responsive.
Hope that helps