First: remember that all Martin guitars up to the 12-fret 000 were designed for gut strings, and used X bracing.
Second: what sort of sound are you looking for?
I've built three nylon string guitars with X bracing that are the size of a 12-fret 000: one with a Red spruce top, BRW back and sides, and 'normal' Martin style bracing, one in BRW/Cedar with double-X bracing (my 'Autumn' guitar, that you can see on my web site), and one with a 'sandwich' top. The 'Autumn' guitar has the most 'Spanish' sound of any one I've made. The Red top, as you might expect, had a more 'forward' tone right out of the box, although I suspect it has mellowed a bit, and the 'sandwich' top had a nice, balanced Classical sound, and lots of it, but also had some structural problems.
The key, IMO, is that if you want to use X bracing, go for the bigger body size, or at least a wider lower bout. X bracing has more strength for the weight, and particularly more crosswise stiffness, and you need to watch the balance to get a good tone from nylon strings.
Second, watch your weight. Nylon strings have a lot less energy than steel at a given amplitude of vibration, and you can only raise the action so much to increase the power. Even for a 000 size box use a top thickness that's about the same as a 'normal' Classical and keep the bracing fairly narrow and not too tall. I'd shoot for an all-up top weight without the bridge of 170 grams or less on a 000 size. Use a low density wood, such as cedar or Englemann.
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