Nothing inherently wrong with "mixing", but this is pretty esoteric construction for a one-off frame. Are titanium lugs readily available? You (or your framebuilder) would certainly want to be completely up to speed on appropriate techniques and materials for fitting, gluing, etc., particularly bonding techniques between titanium and carbon. I certainly would want to be on a trodden path with this, not doing something for the first time. Lugged carbon tubes have been done occasionally in the past, but seem to have completely fallen out of favor vs. molded carbon.

I'm curious why you want to do this. I would think it would be very hard to match the performance, weight, and durability of any number of off-the-rack molded carbon frames currently being sold.

- Mark