First question-the ply of a yarn refers to the number of spun strands it has. It doesn't always tell you much, because an 8ply yarn can be thinner than a 4 ply yarn. Worsted is the more informative word here. Worsted is probably the most common thickness of yarn--about medium. You can go up to bulky and chunky or down to dk sport and fingering weight.
If you're are getting horizontal ridges in your knitting it says that you're knitting and purling at different tensions. I suggest that you try circular knitting for a hat. All you stitches will be knitted and you'll still get stockinette.
If you want to knit flat in st st, you'll have to experiment to see if possibly you're knitting too loosely on one side or another. Some people have to use a different sized needle for the purl side than the knit side.
As for mixing styles, your gauge can change with the style, so if you start a project where gauge is important, you might want to pick one or the other.
The most important swatch measurement is the horizonal one. You can change the number of rows you knit for row gauge, but you can't change horizontal gauge once you start. I'd measure on the st st part, and if you don't have enough to measure 4 inches, measure 2 inches and mulitply.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry