reklein - I'm glad you hadn't mounted those upper topsail yards permanently. I think you'll find that lowering them makes a big difference in the appearance of the finished model. That photograph is a good source of inspiration; notice how the lowered yards on several of the ships are cocked at careless angles. That sort of thing would have been anathema to the captain of a tea clipper, but was quite common in the latter days of the working sailing ship.
I thought I knew the answer to your query about the construction of the Cutty Sark's spars, but it turned out to be a little more complicated than I thought. Remarkably enough the wonderful set of plans by George Campbell doesn't answer the question thoroughly. He does say all three lower masts and the bowsprit are iron, and that the topmasts are wood, but (unless I've missed it amid the huge amount of text on the drawings) says nothing about any of the other spars. The newly-added conservation information on the Cutty Sark's website, however, clears it up: http://www.cuttysark.org.uk/resources/14/uploadedFiles/Conservation%20Plan%201%20History%20of%20Fabric.pdf
It seems the original specifications in the contract for the ship's construction called for the fore and main lower masts to be iron, and the mizzen lower mast to be wood. The latter got replaced by an iron one in 1887. The fore and main lower yards are steel. All the other spars, including the crojack yard, originally were wood - with plenty of ironwork for the various fittings attached to them. During the restoration of the fifties and sixties, all (or nearly all) of the wood spars were replaced. For the sake of maintenance and longevity, steel was used for all the replacements bigger than 5" in diameter, and wood for the smaller ones. The one exception was the jibboom, for which the replacement was made of wood. That's the only originally wood spar that can be seen clearly from a normal viewing angle; all the others normally show up in silhouette. (I think the topmasts and topgallant masts are painted to look like bare wood; all the yards are, and were originally, painted black.)
That material recently posted on the ship's website, by the way, is fascinating and extremely detailed. It takes a minute or two to download, but anybody interested in the ship will find it worth reading.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.