Before you do anything else: figure out what species this sea urchin is. Common names are never sufficient with these animals. I can think of 4 different species with very different temperaments that I've seen sold under that common name.
What kind of acclimation do they require?
LONG. If you want a live Echinoderm at the end of the acclimation, test the bag water and tank water. If the difference is within 0.03 to 0.04, plan on 4 hours drip acclimation (I do 4 hours minimum even when the waters look pretty much the same because I would rather be safe than sorry as I'll explain shortly). I don't like to go more than a change of 0.01sg per hour. If you are unlucky and get something like 1.018 in the bag and 1.026 in the tank, 4 hours may still kill some animals. Some people will say that such long acclimation times are absurd because some people acclimate some animals successfully over a much faster period of time. However, you also risk killing the animal if you guessed wrong and rushed it - and there is no way to fix the situation once that's happened. Skimping on acclimation time with sea urchins leads to far too many posts around the web of people buying apparently active, healthy sea urchins who start losing their spines and eventually die anywhere from a couple of days to a week after the event. Starfish and sea cucumbers suffer similar problems and will disintegrate over the same time period. The exact needs of each species for acclimation are poorly understood, so the only way to be totally safe is to be overly cautious.
Will they do ok in less than pristine conditions?
About the only thing I am casual with for my tanks with Echinoderms is phosphates; urchins don't seem to care at all about that at all. Everything else needs to be as you would expect it in a reef tank. Echinoderms shouldn't go into unstable tanks (the biggest problem with new tanks) and are best left until a system has matured for a few months.
Is there any danger to other inhabitants? ie hermits, emerald crab, snails, six line wrasse, clown fish and green chromis, leather corals, star polyps?
It depends on the species of sea urchin, whether there is abundant natural food in the tank and whether you're willing to target feed it when that runs out. I target feed all of my sea urchins.
Are they good at eating algae?
Depends on the species of sea urchin and the species of algae. I used a couple species of sea urchins (Lytechinus variegatus and Tripneustes gratilla) to eradicate GHA from my 55gal via overgrazing and it was incredibly effective.
What is there specific gravity requirements?
Stable first and foremost! A standard reef sg of 1.025 is fine.
Pros: basically down to whether you like sea urchins or if you're looking to use them as part of an over-grazing solution for algae control (species-dependent of course).
Cons: depends on the species yet again, but collector species will rearrange all the small things in your tank and steal things like glass thermometers, and it can be extremely difficult to regain ownership of some items with some species without hurting the animal in the process. Some other species will crawl off into a hole and you'll never see them unless you look at your tank at 3am in the dark.
Is there a minimum tank size?
Not really, although some species get larger than others. I wouldn't put an urchin in a pico, but that's about it. However, if you expect the urchin's diet to come 100% from the tank with no supplemental feeding, then it's a different situation (but also a bad idea in many cases).
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