Airbus frequently takeoff below takeoff thrust; it is no big deal. At acceleration altitude, the thrust goes up a little instead of down. It is of no consequence to the pilot.
For the failure case, the pilot always has the option of increasing to rated thrust. In the simulator, I frequently increase the thrust after an engine failure, so it doesn't take all day to climb and accelerate.
The engines fitted to the NG I fly are TO 27K, TO1 26K and TO2 24K. We have used 27K once on a very hot day at very high elevation and MTOW. 95% of our operations use 24K with assumed temperature derate. We have a buffer of 6 degrees on the assumed temp to allow for last minute changes to environmental conditions or passengers.
If we can't get the 6 degree buffer at 24K, we increase to 26K and use a higher assumed temperature. This method maintains the buffer for LMCs, but has the same economic benefit as a 24K takeoff with lower assumed temp.
We are often below the minimum weight for 27K.