larrypoe, if your INTEREST is in the little crawlers, there is absolutely nothing wrong with a specialty tank, if you are able to FEED these creatures. Urchins are interesting fellows, if you have glued down your rockwork (superglue) and don't mind seeing a few zoas or a mushroom taking a ride on the urchin. Sally Lightfoot crabs are cute, ditto arrow crabs, but they're problematic in a reef, or with fish and worms, so, say, if you want a neat variety of ornamental featherduster, they're a bad combo; but then so are fish, with those. The starfish are fine, but they'll starve---don't get a linckia, which ARE reef friendly, unless you have a 3 year old reef with about 200 lbs of rock---they eat rock film, evidently, bacterial stuff. But a starfish in a specialty tank is fine if you can keep it fed.
Just about anything is good in a specialty tank: what we try to warn you about here is things with habits not a good match for a reef or a fish tank: the ocean is huge, but in a small tank, they get back to the same spot too often. If you can keep them separate and keep tossing in food, and can keep their water clean---great!
So if you want to start out with some of those 'unkeepables', neat, so long as you know how to feed them. I had a very nice urchin in a reef, and he behaved pretty well (for an urchin) but he grew like a bandit and I had to find someone with a larger tank---amazing how fast and how big some of these things can get. Sally Lightfoots, so small and amusing, can get the size of a dinner plate, and then they want fish for dinner. And if you can get expired fish at the market (Sally'd be fine with that) and keep the water clean, she'd be a very interesting speciman. So don't take anything we're saying here as a wave-off, just a "some things inconveniently eat each other, or too easily starve in a small tank."
And do note that a refugium (often part of your sump/system) if large enough, can BECOME a speciality tank, even a second display, if you set it up nicely.