2/3a's would be too big, they would weigh too much (I think).

If you use a 6 or 7 cell AAA pack it "should" weigh about ~3 ounces. My 8 cell 5/4 AAA pack weighs like 4 oz's so when you figure up smaller cells and less of them I think it should be around 3 ounces.

As long as your carefull with li-po packs you should be fine, their are many precautions you must take but as long as follow the instructions youll have no problems. You would really benifit from using a li-po pack, for one youll save an ounce, and 2 your performance will increase.

If you read into li-po and their dangers most all of the danger can be prevented if you pay attention to what your doing. When I bought my li-po setup the guy told me about ~5 stories of people having them flame up on them, all of the incedences resulted from improper useage, for example charging too long, charging after the pack was damaged, etc.