Matt-
I was going to say that I have had very good luck over the years doing the exact opposite of what you're suggesting...until I reread it. As a general rule I would agree with your statement, I can't tell you how many bloodied yellow tangs I have seen in tanks.
The first tang in my tank was a 3 inch yellow, I then introduced the sailfin which was about an inch bigger. The next one to join the crowd was a 2 inch yellow..and then just a few weeks ago the 3 inch clown tang. There has never been any serious fighting between them, just the occasional posturing, and mostly between the sailfin and the larger yellow.
Personally, I have had good luck adding smaller fish to an established tank. I think it may be because the smaller fish is so intmidated that it never trys to stand up for itself. I also have plenty of hiding room in/among the rocks. I have paired numerous clowns this way as well.
Would you be interested in a 4-5 inch Magnificant Foxface (Siganus magnifica)? I have a local customer that ordered it for a 54 corner tank, I didn't know his tank size when he ordered it. I can bring it to the meeting Friday night if you are.
Marc