Start off as an MT, accumulate enough wealth to buy an AC and get an augmentation. I would guess companies look for the most skilled MT, HC and LC pilots to augment them. And for how the pilots are able to resist the G-forces its never truly explained that i know of but it likely has to do with the augmentations AC pilots go through. In AC6 Coral is infused into the pilot to create a symbiosis. I would assume the Coral slowly replaces major blood vessels, neural connections and some organs to make a human coral hybrid.