Urchins?

Longspines get a little big, plus their spines carry a mild toxin that can be *really* uncomfortable since it's easy to jab yourself on the sharp spines. I would look into a pencil or pincushion urchin as these are a little easier on the hands. An urchin is going to plow through some coralline and will likely munch on any macroalgaes or soft corals that you add in the future, so plan accordingly if you decide to add one.

We've had 2 long-spine urchins in our tank before. They were tiny liverock stowaways that quickly grew from being less than an inch in diameter to about 4-5 inches, including the spines that is. They were very hardy and they did a great job of eating algae but we did get stuck by them a few times and they did get way too close for comfort to our open-brain coral several times that I thought they might pop it!
If you really want urchins I'd suggest the short-spined, common urchin, a plus is that it comes in different colors like white, pink and purple. We've also had 2 of these that we added on purpose, one white and one purple. They are just as hardy but a little less dangerous to your hands in the tank. One funny thing about the common urchin is that they have these really sticky feelers all over their body and they end up getting decorated with sea shells, rubble, plants, sponges, the occasional hermit crab...

I've had mine with my corals. No problem here. I grazes all over the place too. It's a very active urchin. I did prick myself the other day while cleaning the inside, but it only hurt slightly for a few minutes. I think your tank maybe to small for one and a smaller urchin would be the better choice.