Hi bio161,
the Derate is a default reduction in thrust expressed in percentage (eg. 5%, 10% ... 25% is the max) and having his peculiar RTOW chart; the Flexible Thrust is, instead, a way of reducing engine thrust (and save life) by setting a temperature (TFLEX) to meet the minimum TO performance on a "long" runway for a weight lower than the maximum = far more accurate than derating thrust.
but takeoff speeds with full thrust still apply."
Reducing thrust on TO also means reducing the minimum control speeds, thus the most penalizing situation is the full thrust TO, which is always considered by the regulations.
Think this way: V1= full thrust (directly related to Vmcg) / VR/V2 = FLEX
Hope that helps,
fredgrav