[Delphinus]

The answer to your questions I don't think are as straightforward as you hope.

Obviously anemones live on reefs in the wild, so why not in reef tanks?

But then you have to measure out the balance.

Indeed, a BTA will sting anything it touches. Some things may tolerate it more than others. But eventually things like LPS, SPS, will start to recede in the areas of contact.

You generally need a buffer zone around the anemone. This can be a challenge with new anemones and new anemone keepers because if conditions aren't to their liking the anemones will wander.

Each species of anemone has its own preferences as to where they will want to reside. Some like the bottom, some like crevasses, some like sand, some like rock, some like the top. Some are more demanding, some are more forgiving. It's a HUGE spectrum of things to know, but as long as you do your homework and be reasonable with your choices you can have a lot of success in this area. (You can have a lot of stress too, so again the emphasis is knowing what you want.)

Oh and yes, a carpet will eat fish if given the opportunity. Sometimes not, but it's a crapshoot. Basically if a fish blunders into a carpet, then it's game-on. Lots of stories of carpets killing/eating large fish.