i only have experience with Tuxedos, but they seem to be very gentle creatures. He will pick up random loose objects in the tank and carry them around for a while as camouflage, but won’t mess with anything that’s attached IMO. if your tank has no nutrients whatsoever and no algae either, you could supplement feeding with seaweed, but otherwise they feed themselves by scouring your rock and glass for any possible algae. (i had a snail die and the urchin found it, cleaned that shell right out!).
As for BTAs...no coral is completely safe! They can/will move and float around, and will sting and kill anything they touch. Also, to have success with a nem you will want to wait quite a while until your tank is more established, they need a stable environment. I was really tempted by a nem as well (who doesn’t want to see a clownfish hosting one??) but am now very glad i don’t have one in my tank. Don’t worry about your clowns, they will be just fine and are capable of hosting a wide variety of things: returns, powerheads, heaters, all sorts of corals, and every other possible spot in your tank!
