We have a Ruffwear pack for our little dude (70lb Shepherd Mix), it's a four pocket model, orange in color. I don't know how much it usually weighs when loaded, but would guess it's less than 10 lbs, and I beleive 10% of their total body weight is the recommended max load. The pack really doesn't hold more than two days worth of food for him plus maybe his flea and tick comb, collapsible bowl and, occassionally, flip flops for me and my wife, so it would be hard to make it weigh too much more anyway.
When we first got it, we started out with him wearing it empty, then moved to putting rolls of toilet paper in it so he would get used to being wider than normal, and gradually increased the weight from there. He loves wearing the pack becasue he knows we're going on an adventure whenever it comes out and gets loaded up.
Aside from not making it too heavy for them, you really need to make sure it's very well balanced side to side or else it will shift all over the place on them. I always need to make multiple small adjustements whenever we hit the trail to get it sitting properly and settled in. Good luck!