If you're going to knit all rows to make garter st, it's reversible so either side can be the RS. Pick whichever you think looks better. If you're just knitting plain, it should look like it's supposed to within a couple rows. For ribbing, it usually takes 3-6 rows.
I don't try to 'remember' how many rows, I just count them. Some people like to use row counter dealies you put on the needle, but I'd probably forget to click it over. Just easier to count them.
If you're doing all garter stitch, the line of mixed colors is going to show no matter which side you add it on. If you're doing stockinette, the add it as you knit the RS.
A DK yarn is slightly thinner than worsted, sometimes nearly the same, so you have to cast on about 20 sts and see if you can get the same gauge as the pattern with the same size needles. Maybe go up a size or two, and use them if you like how it looks.
A gauge of 6 or 7 sts/inch on size 9 is VERY TIGHT. The usual gauge on size 9s would be about 4-4½ sts per inch. Or else the yarn is too thin, but that should still be about 5 sts on a size 9.
The forum members are very good for helping you, the Glossary link has most terms that are used and many of them have videos. The online magazine Knitty has a lot of articles (listed under What do you want to read
?) that explains a lot of things that you're wanting to know. I don't know of any one
book that explains them.