I am in PT for a ripped knee ligament right now. I started using a bike there, and I have one at home which I use. At PT they have just started me on a stepper as well. My therapist is very aware of PF and has shown me some ways to use the stepper that don't flex my foot. Of course, it's a gym quality stepper with gigantic pedals -- you could put 3 feet on each pedal and still have room. Personally, I think the cardinal rule is -- if you try a piece of equipment and feel too much of a stretch in your foot and ankle -- try something else.
I too have gained weight since the onset of PF. Of course I would like to shed it. But the truth is -- alleviating or (hopefully) eliminating the PF is my first concern and I'm sure not going to do anything to worsen it.