Best cast on method for rib?

I decided to try to knit up a couple pairs of legwarmers for Christmas gifts for friends this year.

Here’s my pattern:
http://www.moonarts.com/blog/free_patterns/pages/aspen-leg-warmers-print.htm

I’ve started twice now, and my cast on edge is rather tight. I tried casting on using two needles thickness, but it didn’t make it much better. Is there a preferred cast on with ribbed work that needs to be a little stretchy?

Also, I decided to knit these with dpns to avoid having to sew. Do I just let the yarn follow up the work as I change colors or is there a better way to do that too? Or am I better off knitting it flat and practice my mattress stitch to hide the threads?

Thanks!