To do air taxi work (ie: single pilot ops) you do not ATPL theory, you can do air taxi work with just a CPL and IR, therefore you could do CPL theory followed the IR thory exams; however it is just as cheap to do the full 14 ATPL exams from the outset rather than seperate CPL and IR theory.
Most single pilot operators will require 700 hours total time with 40 hours p1 multi IFR time (some have a concession for 400) aswell. Although it is not uncommon for air taxi operators to ask for a 1000 hours total time with 100+ multi p1 time.