some vauxhalls (such as corsa B sri) have the opposite to torx for the back brake mounting nuts, its like a big star headed bolt, luckily you can get a regular socket to grip them but its easy to damage the head. i've never found somewhere that sells the correct bits to get them off. so fiats are a walk in the park in comparison.
when pushing in the caliper i've had varying success using 2 tools at once, a vice which you can wind tighter easily (which needs a swivel on the foot touching the caliper) and a big junior pipe wrench to turn the caliper as i tighten the vice. its a right cafuffel but its a method thats saved me more than once. another good one is a piece of wood slightly bigger than the caliper, hammer it in and then turn the piece of wood using a big junior pipe wrench while pushing the pice of wood into the caliper at the same time. its easier than the first method but getting the bit of wood out once you're done can be a bit tricky.