While I did not find out if they fit in the back of the FiST, I do know that they will fit comfortably in the back of a toyota camry. I spent the weekend installing them and I thought I would share my experience.
Since I had acquired the ST3's with the powered drivers base and I didnt want to be stuck with whatever position that the drivers back was in, I decided to skin the seat backs as well and swap the foam to the fiesta back. This wasn't terribly difficult but it did end up being rather time consuming, especially on the first seat. You have to be really careful with the Velcro in seat back to not separate it from the foam. On the first seat I completely skinned the back and removed the foam. On the second seat I only peel the seat back up far enough to remove the airbag, and then pulled the frame out. This saved a ton of time dealing with the Velcro in detail lines in the back. The key to that second seats tactic was learning what I had to do to release the headrest guides, which was use a smaller screwdriver to release a clip on the left side (if looking from the front) near the seat frame to pull them up and out.
I also had to do the same thing to the fiesta seat backs, which was the more annoying of the two. They use hog rings in the back of the fiesta seat, as well as some fabric strap around the airbag that is connected with what seems to be two plastic hook bars with plastic rivets on each end.....I dont know. It was really obnoxious.
On the drivers side I also kept the fiestas lumbar adjuster, which is just a wire the tightens to bring the seat back forward. That only involved cutting out a bit of the foam over the mount, and a hole in the leather to mount the arm.
The seat bottoms were really easy to do, as long as you snip/cut/drill those weird looking rivets out. The seat bottom foam has some loop attachment to the plastic bolster supports that make it pretty much impossible to remove from the support without damage otherwise. With the rivets removed it is trivial to release those loops to free the bolster support. Then you just bolt that to the fiesta frame.
I also made sure to keep all the heated seat bits, not that I saw any real way to remove the heating elements from the seats. I removed the two plugs from from the plug box thing on each of the focus seats along with what I assume is the control that lets the focus have multiple heating settings. I will figure out the wiring some other time.
I also decided that while I was already sweating and cursing with all these seats apart, I might as well put the foam and covers from the fiesta back onto the focus frames. So I did. I had to cut out the foam on the side to fit the shape and size of the focus airbag, and I replaced the hog rings with heavy duty zip ties but since the frames are identical it all went back together like it was from the factory. So I ended up with a set of cloth fiesta st seats on focus frame rails.