1. Yep, use DME the same way. Pour slowly and stir as you go.
2. 2.5 gallons sounds good, but remember the more you can boil the better the finished product will be.
3. I'd steep them for about 30 minutes, get as much sugar and flavor from them as you can. You may want to even add some of the DME to the steeping water to ensure you get conversion from the malts that need it to extract sugars. Throw them all together!