I would watch that thin striped hermit... and make sure you give it a little piece of meat once a week, its VERY oportunistic and will eat just about anything... including snails, fish and other crabs that are smaller than it. I have 3 of them.. and they share a piece of shrimp every week to keep them calm. Also, if you're keeping it in a nano tank... be careful, because they can grow to be the size of a tennis ball. they grow VERY fast.
oh and as for an addition, 2 peppermint shrimp would probably do well in there
watch the snails, because some of them require sand beds, so if you're not doing sand... some of them are not for you. 1 or 2 turbo snails would probably work to start, keep an eye on them though, sometimes they fall on their back and cant get up. then add one more every week until you notice the algae is under control.