Can a Mach1 fit 15in rims in the back?

I don't think so.

It's a requirement to have 17'' rims to put those brakes on, and the clearance is already pretty tight.

I know of some GTs who put 15'' v6 rims on their car with DR and it clears fine because the rear brakes on the GT/v6 are considerably smaller.
 
I have them on mine with the stock calipers and they clear fine(I can't say with the bremo's though)...... 15X8 or 9 with a 5 or 5.5 offset would be best...

You may have to file the banjo fitting a bit but it isn't a big deal....