My 2005 ZX4 ST has been giving me nothing but trouble with vibrations because the alloy rims bend so easily from the Michigan roads. Purchased 3 rims in 2 years. Since I have 4 wheel disc brakes and the alloy rims are 16", can I use the 15" steel rims from another Focus? Concerned with clearance to the rotors and calipers. Has anyone tried it?

Yes the 15" steel wheels will fit just fine. All 05-07 have the same size front rotors, and the rear rotors found on the ST are smaller then the fronts.