Another newbie crochet/knitting question:

[SheilaT]

6. If you mix yarns of different thicknesses,

or even if you mix yarns of the same thickness, you simply use whatever size needles works with the combined bulk of the resulting yarns.

Besides, needle (or crochet hook) size is not fixed in stone. If you use a different sized needle (or crochet hook) than is generally called for, you simply get different results. Although, come to think of it, I'm not sure if you can go down a needle size or two the way you can a crochet hook size or two.

If you are making a garment or anything else for which the resulting size really matters, than you must make a swatch. If you're making a scarf or blanket or anything else where the finished size isn't rigidly defined, it's not quite so important to do.

In the end, don't be afraid to experiment.