You should be able to get even fill light with two umbrellas to the sides, or even a single one (or your softbox) right behind you and over your head. Especially if the softbox is large.
I think white bounce or shoot-through would be good. Just recognize that with shoot-through in a small white studio, a lot of the light is actually bouncing backward and off those white surfaces and providing all kinds of fill. Outdoors that reflected light will be wasted - at the very least you may need higher power settings than usual.
You'll definitely want to use a sufficient aperture to get everyone in focus. f/11 might be the safest bet. Personally I find it easier to bring in ambient light with higher ISO's - 400 or 800 maybe. On the 7D I don't find noise to be objectionable at those levels but try it beforehand and make sure you agree.
I think your safest bet on white balance would be shooting a custom one. Let me know if you need help doing that.
Another helpful technique for you to look into is Neil van Niekerk's technique of metering with the histogram. Check out
http://www.planetneil.com and read his posts on metering. Or you can just break down and get a Cyber Commander or other light meter!