Once I used a mix of 3 ATFs on an old car with a perfectly running transmission. It immediately started slipping on shifts. I don't know if the problem was due to any specific ATF used, or the specific mix, or perhaps the old transmission not taking the new fluid kindly. To be safe, I wouldn't mix unless topping off the ATF. By the way, the Mobil 1 synthetic ATF almost certainly can be used as a power steering fluid. Most PS fluids are basically a less stout version of ATF fluids. If you can use it in PS, you can just suck old fluid from the reservoir then top it off with Mobil 1. Repeat the next day (it will take 4-5 times to use up 1 quart in most cars).
PS: sorry didn't see the above comment about Honda's. Any Mercon, Mercon V comparible or better ATF can be used with no worry as PSF in Fords for sure.