UK: I've been working on dragons for the better part of the last week (a few hours at a time here and there) and I've found out a few things. I was playing around with the ACs, trying to get them to a nice progression (to fit everything else, since everything scales), using 10 + size + NA (3-4 + 1/2 HD)+ age category, and I discovered that:
The sizes they have listed for dragons are not all correct. I had to tweak a few to get them to fit properly - for example, great wyrm gold, red, and silver dragons (40+ HD) are Titanic, not Colossal.
Your natural armor bonuses based on size are off. I was banging my head on the desk, trying to figure out why some ACs worked and some didn't, and then I tried changing the NA size mods - and they all fell into place.
What they should be: Large: +3, Huge +5, Gargantuan +8, Colossal +13, Titanic +18, etc. Basically, it's the size mod (-1, -2, -4, -8) * (-1) + 2, 3, 4, etc.
A dragon's AC should be HD + 11. I didn't discover this until I was halfway through adjusting for the new size mods, but after that, it made things a lot easier and enabled me to see that some of the sizes were indeed off (and helped me prove that golds, reds, and silvers were indeed size Ti - I couldn't believe it myself).
And finally... I decided to try this out on one of your dragons. I chose the Platinum because it was the first one I found that was semi-normal. What I came up with...
HD increase by +5 (starting at 29), not +6.
The size progression goes C, C, C, Ti, Ti, MF, MF, MD, MD, MT, MT, MS.
The deflection and divine bonuses are correct - the end ACs start at HD + 16 and increase by +1/HD.