Back For Solid Gold Advice!

[sintaks]

Hello all... It's been close to a year since I last posted on the forum. You all helped me make the choice of putting my first puppy Sampson, a Golden Retriever, on Solid Gold Wolf King (Thanks Prin!) Well, I'm at it again! I just rescued a 13 month old Golden/Lab/Chow mix named Selma. You'd think with those big breeds mentioned that she'd be a big girl, but she's not. She's just a little thing. Maybe 40 lbs... haven't weighed her yet. At any rate, onto the questions:

1. I'm wondering if I can feed my new girl Selma Wolf King as well, even though she's a smaller dog. She comes from big breeds but is MUCH smaller than Sampson.
2. Do you think the 2 cups I mentioned above is too much for Sampson? Occasional soft stool, but mostly good. He's 76 lbs and VERY active. Perfect shape and no health issues.
3. I'd like to feed them the same food. If I can't feed Selma Wolf King too, what food would be good to transition them both to? Hund-n-Flocken? Maybe Barking At the Moon? The shelter had her on Purina One Lamb and Rice.
4. Is transitioning hard on a dog if you mix foods for a few weeks? I certainly don't want to hurt Sampson.

Any help would be most appreciated. You guys have always been the best source for me to make wise decisions about my puppy's health.