Not advisable -- you put the reasons out there yourself.
Sebae anemone is fragile, gets very large - too large for a 29. Clarkiis will host in just about anything, from my experience, they'd do just as well in a BTA in an appropriate tank. Have even seen them host in a Condylactus!
You don't want to put multiple species of clown in the same 29 g tank, that's a recipe for a fight.
In my experience, Clarkiis are no more aggressive than average. Maroons, Cinnamons, Tomatoes tend to be the most beligerant, in my experience, Clarkiis are somewhere in the middle. I've found skunk clowns to be about the most docile, but I've had one for about a year in my sell system that has become a bit of a tyrant. She doesn't look enough like "Nemo" to be attractive to customers, but she's a pretty fish, not too big (they are a smaller specie of clown)... just doesn't have the right stripes
I'd forego the temptation this time, if I were you.
JMHO
Jenn
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