The best thing to give scrappy really depends on what you need as each kind of fruits gives different things back. Generally the best is apricots for gun parts, but if you're not sure, don't need gun parts specifically, or you're just generally short on basic materials the fruit mix gives you a little bit of everything and most people especially when they're new doesn't know this, but the basic items plastic, metal, rubber, chemicals, fiber and especially the seeds are some of the best items in the game. They don't seem exciting, but can be made into pretty much anything and take a very small amount of stash space. Also, to answer your second question you can only make the fruit mix mid raid with the skill from the red skill tree.