Is there a rule? Well, the rule is to use the right right pattern to work with the gauge of the yarn. Meaning, if I'm using a very fine sock yarn, maybe 9 sts to the inch, then I use a pattern with that suggested gauge. There are a number of patterns for worsted weight. Here's a free one for 6 sts per inch: http://www.lionbrand.com/patterns/60049.html?noImages=
. Here's one for 5 sts per inch: http://mary4169.tripod.com/Easy_Worsted_Socks.pdf
As for the content of the yarn-- yes, having some nylon or other acrylic in it does make it much harder-wearing and will give some other fibers, such as cotton, more elasticity. But having said that, I've made socks out of pure cotton that worked because I wore them in boots, so they were held up against my feet, and also because I liked the slouchy tops.