I can't give a comprehensive answer as to how we hire, but I can offer my personal experience: I was able to finish my last year of part-time law school when I started at 18F. I was working full time, and everyone was very respectful about having a life outside of work.
In the shorter term, do check out https://micropurchase.18f.gov. It's where we post very short term contract opportunities ($3,500 or less, and tasks typically take up to a week). I also happen to be a dev on this project, so feel free to reach out here or at [email protected] if you have any questions.
I'd love to get involved, I'm mainly focused in software engineering. It seems like a great cause to partake in.