There are rules? The half and half guy was my first reroot and going outside in just made sense to me. It makes planing stripes and patterns a lot easier. You can start from anywhere on the head. Don't skimp on the thimble, get a good thick one.
Another place to get hair is party supply stores, or the party supply aisle at Target. I picket up 6 piece clip on neon braids at Target, I guess 12-18 braids per Barbie depending on length and up to 24 for a Monster, and have used the little clippies to help keep the hair out of the way. I haven't figured out how many of the cheap extensions at the dollar store you'd need.
After awhile you'll be able to grab a bunch of hair and estimate if it's enough for your project.