I'm in that reserve unit out of march, and there are some of us younger pilots that do fly for regionals as well as do the reserve thing. Baseops has a good bit of info for the specifics on what the process is to get a flying spot in reserves or active duty. But, before any of that you need to have a 4 year degree. So to answer some of those other questions you have....
If you washed out of training, you would most likely be released from your commitment ( i think), or maybe put on inactive status We dont' have much need for you if you can't fly for us.
One of the many benefits of the having the military thing is that if you should come across some rough times at your civilian job, you can usually find a little extra compensation with the military flying.
If you come to fly for the reserves, you will fly whatever that unit that hired you flies. So, obviously you'd want to do some research. The reserves works a little different than active duty, so again do some research on that topic. You apply and interview at units that have the type of airplanes that you'd like to fly, and are in the location that you'd like to live, because when they hire you, that's where you'll be (in most cases).
Any other questions, ask away...