That's a bulky yarn indeed! I've never knit with yarn that heavy. I wonder if the weight and thickness of the garment will be problematic in getting the leg warmers to stay up on the leg....?
If so, you can try threading elastic thread through the top three inches afterwards. Just skim the thread through the back of the knitting. You can also knit it in with the yarn, but by skimming, it's easier to tighten it fo your liking.
As far as making it the right gauge...
One common problem, is if you make your swatch on straight needles, knitting back and forth, and then make the object knitting in the round, you can end up with two different gauges. This is because many (most?) people knit at a different gauge than they purl. I've heard it recommended that you knit your swatch in a circle, like make a sample hat, instead of a swatch! I, of course, have no patience for such a thing. Luckily my knitting and purling is consistent enough that I can get away with flat swatches. But this may not be the case for you!
Another suggestion is to double check your gauge on the project itself as you go. A lot of knitters loosen up as they work, and this can make your project get bigger.
I assume you're doing the top few inches in ribbing? K2, P2 is the most elastic ribbing--that is, it pulls in the most of any ribbing.
Good luck getting your gauge consistent!