In regards to Kroot and other things, most psykers have their own methods of training and shielding from the warp. To my knowledge, Kroot are not inherently psykers, but can become such by consuming other psykers. Therefore, kroot that fight against the Eldar a lot will tend to have psykers among them. How they maintain stability and don't go bonkers? Good question. The greater good, perhaps?

More than likely they are insignificant enough that it doesn't matter? That or perhaps psyker kroot get some serious problems.