Does being lighter gives someone going through the pararescue pipeline an advantage?
It helps with running and cals but just make sure you don't neglect your weight bearing activities such as rucking and buddy carries.
Is it better to have a little fat or be as lean as possible going into selection?
A little fat is totally fine, all anyone really cares about is whether or not you can perform. From what I have seen people with no fat have more trouble than those with a little fat.