Said it before. Never a good idea to mix loads in the magazine. The pros don't do it and neither should you. It will affect your terminal performance, and may cause legal problems in the aftermath. The best in the world will get confused about which round is "up" and ready to go. And confused is something you do NOT want to be in a HD scenario. And confusion is the very devil to explain later, confusion is also a cornerstone of negliegence as defined in civil law.
Just use what you think is best for your situation and do not get fancy. Simpler is ALWAYS better where firearms are concerned. By the way, for HD, buckshot is almost always best, with slugs way behind except for open area scenarios, and birdshot is always for the birds. While I am ranting about ammo, never but NEVER, use any less lethal ammunition for HD. If you are morally incapable of killing someone, get a rolling pin, you have no business with a firearm. Just think about it, when the Abrams tank was being designed, there was much hand wringing about whether or not to make it a semi automatic gun. It was decided that the main gun would be a single shot because with the best trained crews in the world, there was concern that someone might forget which ammo was next to be fired-and this is with 3 highly trained professionals doing it!
OK, EOR (end of rant).