One must keep in mind that derate and flex should not be confused.
(a) FMC selected derate, in effect, is the same as hanging a different engine on the wing for the particular flight. The derate thrust level becomes the maximum thrust level available for scheduling during that flight
(b) flex thrust (typically using the assumed temperature technique) is a means of limiting thrust for a particular takeoff, but with the option of increasing thrust (if the crew so desire) up to, but not exceeding, the present rated thrust (which may be a derate rating)
(c) overboosting beyond the takeoff rating (whether such rating be the maximum or a derate setting) can compromise things such as speed schedules (Vmca and Vmcg considerations)