They are basically "screwed" back into the caliper, not compressed back in like the fronts would be...There is a special tool, but if you're careful, you can use a pair of vise grips or channel locks to screw them back in....The item you'll be ruining if you don't take your time is the dust boot for the caliper pistons...It's definitely do-able, just be careful and take your time.
Yes it's the same fluid but the fluid accumulating in the calipers gets rust flakes and gunk in it and according to brake industry experts, if you let that get back in the lines it can make its way back to the ABS control valves and result in costly repairs. Now whether or not that's a reasonable risk is up to you. Opening the bleeder is a cheap way to make sure it doesn't happen.
I had the same problem - compressing won't work. When I learned that they twist in, I pushed back the rubber boot and put some pliers around the round cylinder. Then I twisted for days until it was full in and the rubber boot wouldn't push back any farther. Don't forget that the V-shaped cut out on the cylinder needs to line up with the pin on the brake mount (sorry, I can't remember the tech names right now) so that they assembly will slide back in. I had one side where the pin wasn't going into the V-shaped cut out and no matter how hard I pushed on the spring-loaded assembly, it kept clinking on something. That something was the pin that needs to fit into the cylinder.
(Perhaps someone can explain that better.)
Disclaimer: I had the caliper completely off when I twisted the cylinder, so it wasn't really that hard.
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