www.guitarbt.com isn't bad and this site has a few jam tracks too. On the main page here just click on Jam up there on the top.

For your first question I usually use alternate picking. It's just become habit for some reason. I say learn and use everything you can. Why limit yourself?

On the second question, it is tough to sweep but entirely learnable. I get better slowly, and every now and then I get a good one but not often. The biggest part of sweeping really ain't the picking so much as the muting. All you gotta do when you pick is rake the strings in the same manner you would a chord but for lack of a better word "slower". Tilt your pick a little in the direction you are sweeping. Muting is tough and getting the timing is tough. This site has good sweeping lessons. You probably need highbandwidth so if you are determined to learn i think you should sign up.