Thank you.
Stripped them left calliper seems to be fine, right one has uneven wear, although the pistons move freely enough so need to investigate further.
How far out do the piston go before they fall out of the seals?
The only safe way to answer that is to hold 3 pistons back at the same time by whatever means you have, i have some small g-clamps that are really handy, SLOWLY pump the lever then as soon as you see a clean shiny part of the piston then that'll be enough, if you're cleaning them up whilst still attached.
With the hardest toothbrush you can find, use ONLY brake cleaner, then slap on the red rubber grease so that every part of the exposed piston has grease on it. Then gently push the piston back in and move on to the next one, clamping the other 3 again. Leave the other caliper on the other disc while all this is going on.
Mine are generally clean and move freely because of the way i do this every time but if you have iffy looking rust spots or they generally look shite then you might have to get a bit more involved, as in a proper strip down and clean up, or maybe even new seals.