This morning I decided to just go ahead and do it and go from memory. In total it took me 6.5 hours for both sides. Pretty decent I would think. 😁
I did find a few problems. On the driver's side, the clip that holds the inner brake pad was missing. And i didn't have a spare so installed the brake pad without the clip - making sure it was snug against the brake cylinder when I installed the caliper. Also, there was a tiny tear in the piston seal. I do have a complete rebuild set, but rebuilding the rear caliper is something for another day/year/decade. What I did is actually glued it together with a tiny bit of super glue. Enough to make sure no dirt can get in.
The sliders of the same brake were seizing bad. I was already planning on cleaning them up and re-lubricating them, but getting them out wasn't easy. I used a flat end screwdriver and tried to hammer it out but that actually caused the bolt to get out of the slider (and destroying the bolt). I eventually got the slider out from the other side (using a socket wrench that fots the hole, and a C Clamp). Fortunately, I had two spare sliders so I threw the old ones out and used the replacements.
The last time I had the rear brake pads replaced, I had it done by a garage. And I noticed they replaced the locking nut (on the actuator screw) with a "normal" nut. Apparently that didn't really cause a problem these past years, but I know these locking nuts are "special" in some kind of way...
I used a iron-saw to change the hexagon levers into octagon levers to make my live easier installing them.

I did a test drive and my Fiero actually brakes better now. Although I can hear the pads rubbing against the rotors (can't remember the english word for that, but you know what I mean. "stick?").