Here is my experience with a pair of Tomato Clowns...
The were in a 30 gallon long and had the whole tank to themselves for a few years. I have added coral and BTAs without an issue. As soon as I added anything else, they got destroyed. Those clowns showed themselves to be very nasty.
They killed off a pair of Damsels and a Valentino puffer on two separate occasions. Again, I'm speaking of Tomato clowns which are very territorial and can be fatally aggressive toward new inhabitants.