as bisch stated, "skinny" is subjective. 6 strands of b55 is close to the same size as 12-14 strands of bcy X.
As far as the nock fit goes, that is one of the many reasons they make serving sizes all the way from .007 to .032.
Personally, I have had really good results with 12 strands of BCY X, and I have not heard anything about the strings stretching more than any other string.
Order from someone you trust, and then trust them.
Maybe one of my customers will chime in and give you some feedback about stretch. I do my best to prevent further stretching. I have found when letting a string stretch at higher poundages than you will ever get from a bow(#200 or more pounds), the materials will get to a settling point to where they don't stretch anymore. If it doesn't stretched with #150-200 of pressure on it, why would it stretch more with #50 on it? (just thinking aloud).