I was under the impression that DEX VI certification was more about strict adherence to a specific formula of base oils and additives than meeting bench test specs.
Mixing ATF....that's a good question. Under most circumstances, they "should" be mixable....but I don't know if I'd go mixing a boutique like Redline or Amsoil, with Valvoline, SuperTech, etc...you pay for a good boutique, may as well "top off" with boutique