I read somewhere that a lionfish can eat prey that's nearly as large as itself. Not only can its mouth expand, but its stomach can expand 30x its normal size. In other words, it's probably best to have clowns that are larger than the lionfish. Fu manchus only get about 4", so you're still running the risk of the clowns getting eaten. You can start with a really small one, then swap it out as it gets bigger.