I've never heard that polymnus don't host the same anemone for they're lifetime. Where did you hear that? I would think that the only reason they are apt to up and leave the anemone would be if they found a better or more suitable host, which is entirely avoidable if you only have one anemone. My ocellaris stayed in the magnifica i had for 6 years, then upon introduction of my mertensii, they left it for the carpet. But with one anemone you won't have a problem of them leaving the haddoni.
Also, on the point of saddleback's being hard on anemones; If you do end up going with this species of clown, add the anemone first, and give it at least a month to settle in and get comfortable. Also try and get a large carpet, or really small clowns. If your anemone is a good size bigger than the clowns and it is already established in your aquarium, you should have no problems with them being to rough on the anemone, i never have.
fwiw