JAR-FCL 1.355
Before being permitted to begin an approved
course of training for a FI(A) rating an applicant shall have:
(a) at least a CPL(A) or completed at least
200 hours of flight time of which 150 hours as
pilot-in-command if holding a PPL(A);
Thus if you have a CPL you may commence a FI Course.
The 100 hours you refer to is for the issue of a CPL JAR-FCL 1.155 c)(1)
(1) 100 hours as pilot-in-command, or
70 hours as pilot-in-command if completed
during a course of integrated flying training