I keep lots of anemones, in tanks large and small. Here's my take:
1) If you have a powerhead and an anemone in *any* tank, put anemone guards on said powerheads. Anemones love to position themselves next to a current and when on the move they'll head straight for a powerhead. As others have hinted at, anemone puree is lethal to pretty much everything in your tank.
@grizfyrfyter 3D prints anemone guards for a very reasonable price.
2) The risk is always present that the anemone will sting and damage or kill something. That said, I've had great success getting them to stay put. What you need is a rock with deep crenelations in it and a current flowing nearby that the anemone can extend its tentacles into. They do
not like current blowing on their front, on their body or on their foot.
3) Stability of parameters and pretty clean water are important for anemones. The smaller the tank, the harder this is to pull off. The smallest tank that have kept anemones in was a JBJ-24 all-in-one. I had the hood on and a Tunze nano ATO, so salinity stayed bang on where I set it. I converted the middle rear compartment into a refugium and did a 5-gallon water change weekly. As a result, nitrates were consistently ~ 0.2ppm. I still have the rose BTA that was in that tank, so I consider that a success.