Hueco, paranoia is good in reloading, it keeps you safe. But you may be getting a little too paranoid.
Carlyle said to start below maximum and work up. You don't necessarily have to start at the very minimum once you have an idea how your gun and your choice of powder/primer/casing work together. Yes, you can interchange any magnum primer with any other magnum primer. However, when you change
any component, you should reduce your current load by at least 5% and work up again.