I probably should have clarified. Soma, as Masa pointed out, is designed to better endure space, and that primarily includes g-forces. By going through the course at maximum speed, she was undoubtedly enduring heavy strain. Sergei could probably use her mobile suit, but certainly not anywhere close to the speeds and maneuvers she was going through. He'd certainly red-out, and probably black out if not kill himself. He commented not only at how fast she was going, but also how accurate she was.

No, I don't think that all Enhanced Soldiers would have the same response to each other. But whatever nanomachines are in Soma certainly reacted violently to H/Allelujah's proximity. It was a nasty feedback loop, like when you put two phones together. They both ended up in each other's head. H/Allelujah was able to endure it by switching personalities, while Soma was overwhelmed.

We only know half the story behind what just happened. We know most of the facts of Soma, but still nothing about H/Allelujah. Why does he have two personalities, what causes it, what was with the flashback of him and someone else adrift? He may be an Enhanced Soldier, he may not. It could be something entirely different. But...for whatever reason, Soma and Hallelujah connected.