Doing what you intend is simply undercutting people who invested the money and time in completing there CPL.
Undercutting implies a lower cost. From scratch, the cost of becoming a FI with a PPL is likely to be the same as becoming a FI with a CPL. The extra 50 hours PIC required by a PPL holder will balance out the cost of obtaining a CPL. Add to that the fact that after 3 years, any future advances will require a resit of the exams. The pay is likely to be the same so the PPL holder will be less likely to move on, making them more attractive to PPL schools. No undercutting involved!