I purchased 600 sqft of used oak flooring to match existing oak flooring in home. The new (used) flooring came from what seems to be same origin. Here is the issue: some of the planks (~40%) are a few thousandths of an inch thinner. The thicker stuff seems to have been under carpet so it seems the thinner stuff must have been refinished once or twice. My question is should I plane down the thicker planks prior to install? Or install the flooring then sand everything at once. I am planning on sanding the entire floor (existing and new flooring) once completely finished then stain and finish.

I am leaning toward planing the thicker planks prior to install but want some other advice.

Waste of time to plane it prior to install, plus fairly inexact. You will not save any time by planing and you could go too far and ruin the planks.

Go ahead and install it and then sand. Really the only way to do it but if you factor in that you were going to sand anyway then makes the decision easy.