Octaves: The coefficient of elasticity affects the tuning. The string may be fine--or it may be false, or a combination.
Do try the renaissance trick of flossing it in the light. The easist way to remedy the octaves is to vary the tension, at a certain point the "stretchiness" of the strings gets a bit closer. Calculate the tension of each string using Arto's indispensible string calculator. If you are set on that combo of strings, you are limited to varying the thickness. If the strings are too dissimilar, they won't really get into tune. Also, if there is a substantial difference in thickness, one is bending more than the other, so you may need to regulate the action to compensate. If you took a paired course and put one good nylon and one good carbon together onm the same note, they would go more out of tune as you go higher up the neck. Similarly, on a baroque guitar, if the bottom course is nylon and the third course is carbon, the tuning between these two courses will drift. dt To get on or off this list see list information at http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html
