1 1/2%-2% for maintenance, so 3% would be a good start for weight gain. You can adjust as you go, depending on your dog's metabolism. For example, I am feeding 73lb Conner as much as I fed 90lb Fletcher because Conner has a harder time keeping weight on. I feed thawed food, some people like partially frozen. Choice is yours, and what Ruby prefers. Pork can be a little rich, but I have a friend whose dog is allergic to beef who feeds mainly pork with no problem. If you can find baby back ribs on sale that would be a good start, ribs are pretty soft bones. Pork necks are also an option, but both will need some more meat added. Fish is fine if she will eat it. Some dogs love it, some don't. Small fish (sardines, etc) can be give whole. Skin, bones are fine.
It seems complicated at first, while you are fine tuning what your dog prefers and what works for you, but it really is easy once you get started
and yes, you can mix kibble & raw!