I think if it's a 1.1 or 1.2 you wouldn't need to disturb this bolt for just a pad replacement.
The caliper body hangs on the mounting bracket by a bolt at the top (the new one you have) and a pin at the bottom.
Look at this one, the bolt would be to the right (where the red line ends)
The lower pin and R clip can be seen on the left.
http://www.ebay.co.uk/itm/Fiat-Punto-Panda-Seicento-Passengers-Side-Front-Brake-Caliper-1998-2010-/181125009998?pt=UK_Cars_Parts_Vehicles_Other_Vehicle_Parts_Accessories_ET&hash=item2a2be4524e
If you are just replacing the pads, all you need to do is remove the tiny R clip that stops this lower pin from sliding out.
Pull/knock the pin through and swing the caliper up to get at the pads.
You can discard the new bolt as you aren't removing the old one.
If you were doing the discs as well, you'd need to remove the caliper body AND mount so the disc can slide out, then you'd use the new bolt, they are pretreated with thread lock to stop them rattling loose.
As a guide these bolts aren't too tight, around 30nm (22ft/lbs) would be enough.
