Actually if a previous owner ever put in a new caliper/calipers he could very well have a hex head back there. I've bought reman'd calipers and one came with a hex and one with the T50 torx. I just re-used the one good torx I managed to get out without stripping/torching to keep it all the same. It's probably a T50 though and be VERY careful when taking it out, they have a tendency to be stuck and not want to come out. Heat helps here. If you blow one out the replacement size is M10x1.5x80 and you can use an allen head. You'll likely have to order one online so this is more incentive not to booger one up lol.
And yes, the rears on this car do not compress with a big honkin set of pliers (or the actual correct tool but I've literally NEVER see anyone use one), you can buy a little cube with a bunch of nubs that goes on the end of a 3/8" extension from any chain store. Turn the piston clockwise to spin it back down into place, it should sit ALMOST flush but not quite with the rubber boot. If it doesn't then bring it back out all the way and go in again, keep at it until it sits down in there. I had an epic battle with my brakes upgrading to the NPGs out back, its documented on here lol. A stock brake job should go quite smoothly.