audio reprocessing for machine learning
The decision on which method to use to scale the input is very much determined by the objective and therefore what follows the scaling.

A simple linear scaling (whether peak, minmax or other) propagates to the rest of the processing chain as a multiplication. But anything that affects the dynamics of the signal (how quickly it rises and falls) ultimately shapes its content and might affect the classification results. I am assuming here, that this classification is based on some sort of spectral features (?)

The point is to look at the feature space and make sure that it is more or less spread uniformly across its bounds and representative of each class. Any improvement brought about by scaling of the original dataset would be assessed on this basis.

If your original material is something like a talk show, or other occassion where there is clean speech and no overlap of speakers then plain linear scaling would be fine or not even required. 

But if you have a heated up discussion or other "quirky" content where people might be coming near the microphone or there are shouts or unpredictable background noise or people talking and overlapping, then that is much more difficult to resolve. A compressor could help up to a point but you may find that you have to provide more training samples to the classifier anyway, so that it learns that "Calm speaker A" and "Shouting speaker D" is the same person (i.e. scaling would not help much there).


  ...any suggestions on declipping?


In reality, if the audio clipped the information that was to be recorded at that point is entirely lost. And it should be treated as entirely lost.

But, if you have a huge amount of data and not quite but somewhat clipped audio. Maybe it was clipping but not all the time, maybe it was saturating and popping but not entirely hitting the rail and staying there for 5 seconds (for example),....then it might be possible to treat this as a regression type of problem and extrapolate what the clipped part was supposed to look like. This however is easier said than done because there are a lot of parameters that affect the performance of the declipping. For more information see here.

Hope this helps.