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SL's get pretty big. I was in the LFS the other day admiring one of the tanks and a 5" SL came crawling out from behind one of the rock..... I took a step back
It looked like an underwater tarantula crawling around.
About the only crab I'd look at would be an Emerald, and even those get decent size, especially for a 6g. If your looking for a little extra activity and scavenging, a few small hermits would probably be the way to go.