hmmm u can't put your anemone there, it would move there.. but if it does move to where ur leather or mushroom is, most probably ti would move away... well that's what mine did in the past anyway.. they do sting actually..
it's dangerous in a sense that it would move arnd and topple your corals... other than that, will be the stinging.. not really that nice to have an anemone in a pure reef tank.. looks wierd unless u get a really small one for an even smaller clown..
a dying anemone will have tentacles that are pointing downwards, shrivled up for many days.. or it would just not attach itself... as in rolling all over the place.. when it becomes white, means it's bleached (lack of lights, bad water para)
ermmm i can't ans ur last question.. i don't know much abt anemonies but the common ones u find at LFS sould be fine i guess...