Can you mix mercon and type f transmission fluid together to change oil in my ford trans.?

No, it is not a good idea to mix Mercon and Type F transmission fluids. They have different friction modifiers and are for different purposes. If you have a 1998 or newer Ford vehicle, you should use Mercon V; earlier models require Mercon IV. You can convert a Mercon III/IV vehicle to Mercon V by having a complete flush done at a competent transmission shop.