Pregabalin may need a ramp up period, but all other painkillers should work immediately. If anything, their effectiveness diminishes over time as your neuro receptors adapts to them. This is especially true for opoids as dosage has to be increased to maintain the same effect.
Ibuprofen or any anti-inflammatory drug will theoretically slow lengthening, since you need inflammation to increase blood flow to the injured area (your osteotomy). By reducing bone growth, you can lengthen slower, further reducing discomfort. However consudering the average femur consolidates at 0.25mm per day, it should not be a problem unless you grow twice as fast as the average person.
If you want maximum length, there are many factors you should consider
1) Longer femurs are more susceptible to fractures due to torque. It's easier to break a long tree branch than a shorter one.
2) Your long femurs will be very noticible past 6cm of lengthening, especially if you were already proportional pre-operation.
3) The most important aspect is soft tissue recovery. Like any rubber band, there is a finite length that your muscles/tendons/fascae will stretch. You risk permanent impairment if you lengthen too far. Stryde caps at 8cm for this reason. I personally may consider a max length of 9cm on Betzbone, even though the cap is 12cm.