Not sure about the food mixing issue in terms of keeping meat and veg separate, or eating fruit by itself. But as for the raw/cooked combining issue, the WAPF recommends that as often as possible, your meals consist of something cooked, something raw, and something fermented. (Some people call fermented veg and fruit "super raw," because they actually contain health/nutritional benefits above and beyond raw...aside from the live enzymes there are probiotics and the bacteria actually increase the content of certain vitamins.)
The book Enzyme Nutrition by Edward Howell is awesome. A lost gem from the old days. He does a good job of explaining that eating at least some of your food raw is very good for digestion. The stomach consists of three sections. In the upper section, not a whole lot of digestion goes on. The muscular churning (physical digestion) and enzymatic (chemical) digestion happen lower down. When food comes into that upper portion with its own enzymes still intact, they essentially start to "pre-digest" it.
I'm not explaining it very well, but he does in the book, and it makes a lot of sense. (Much more than what I've explained poorly.)
As for salads, like Lindsay suggested, salads used to proceed meals because they were made up mostly of bitter greens, which stimulate digestive juices. Not sure the same holds true for iceberg lettuce doused in thousand island, hehheh.
I don't think there's a whole lot to the "no fruit with meat" thing. I've heard a lot about fruit staying in the stomach or gut longer and fermenting, or the protein putrefying. I dunno. Call me crazy, but come fall and winter, there are few meals more enjoyable than something like a pork loin stuffed with apricots and raisins. Or how about beef roasts with blackberry sauce? Brisket with carrot tzimmis (usually contains raisins, prunes, or some other sweet, dried fruit) on the side for a Jewish holiday. I feel like there are a lot of classic dishes that combine meat and fruit. In tropical areas, don't they do a lot of fish with fresh mango salsa type entrees? Not saying these are ideal for human health, but human society/culture would be missing something without them.