by just pulling the pan your only draining about 6 qts. from the tranny, out of the 12 or more thats in the Tq. Converter.. etc. i dont know how well mixing 2 fluid brands will be. i personally would use Motorcraft Mercon/Mercon V, since normal Mercon was discontinued for Mer. 5. just my 2 cents
Usually at work we suggest a flush every 30k or so. We we don't do many filter changes. We hook a flush machine in the cooler lines and push 16 new quarts in and if pushes the old back out into a bladder