I am a newbie to this forum, but a PPC/Bullseye shooter for 20+ years. With regard to mixing cases while reloading, different manufacturers may have varying sized flash holes in the cases. The different sized holes will result in slightly different burn rates that will have an efect on your load.
I used to de-cap the cases, clean and tumble, the check the flash holes size with a small drill bit. Sort cases with the same size flash hole or, if you have the time, drill out the hole of your cases to match the largest size flash hole in your batch of cases.