I think most of the carpets that you'll see in the LFS will be larger than 10 inches when fully expanded. I've seen quite a few gigs in stores and only one time did I see them at about 5" and since they were still acclimating I'd guess they were 3/4 of their size. I've seen more smaller haddoni. I'd suggest a magnifica -- though not a carpet -- since I oftentimes see smaller ones. The short story is that you're probably going to get a lot of heat for trying to house any carpet in a nano, but it's nice to know that you have a place to put it when it outgrows its current digs.
But, as others have mentioned, to answer your original question -- typically get the clown first then add the anemone later -- but it really doesn't matter if it's an established tank. For example, some clowns beat up on new anemones so it may be better to add the nem first, let it settle in, then add the clowns.