I don't have experience with this yet (I will be starting research and lab work this summer), but I can answer your second question. Basically, I just e-mailed my advisor and asked if he had funding available for me to start in the summer. I brought up some ideas on how to start my thesis research, and mentioned that it would be a big help to have another field season.
I've heard mixed things about starting in the summer too; on paper it looks good, especially for fields where season matters (soil science, ecology, hydrology, etc.) and the fact that you're getting an additional summer is more important than the three extra months. I might be a little different than most grad students, as I graduated with my B.S. last fall and have had this semester off, so I've had my break and my time to relax, and now I'm eager to start research.