We have two long spines, and a pincushion. They do eat coralline, but it grows back. They do occassionally knock things over, but only when I havent' secured them properly
One way I found to lower their coralline destruction is to feed them. All three of them LOVE freeze dried krill. The long spines especially go nuts for it, they will "run" across the tank at feeding time, and plant themselves at the top center of the tank with spines out of the water, waiting to have krill dropped onto them. They then work it into the middle of their back where their mouth is. They munch on it for hours.
Great creatures, and a lot of fun to watch. They also add a certain "Wow, that ain't no freshwater tank" mystique to the aquarium.
As far as their toxicity, don't get poked with the spines, trust me on that one. I have read that if they die, they can be toxic, but again, in a larger system, I wouldn't worry.
I had to move one from one tank to another, if you ever have to do this, a spagetti strainer and a tupperware work well. Scoop him underwater with the strainer, and 'pour' him into the tupperware to travel.
We have one that is 3, one is 2 1/2, the other about 6 mos. old.
Here's the oldest: