i thought that they made a point that Soma's body was built to have better performance in outer space. since she's also been trained since childhood to be a soldier and to bring out the max potential from her genetically engineered, nanomachine enhanced body, I'd expect someone like Sergei, who appears to be a normal human being born on Earth (his remark about preferring gravity leads me to this idea), would not be able to pilot Soma's unit with the same results...

Sergei might be able to use it, but it might put too much strain on his body... Or he might only be able to use it clumsily (think of a noob playing a FPS with high sensitivity settings)

you're right, I never expected Allelujah to be an enhanced soldier until this episode (Lockon's sniping abilities seem abnormal, and Tieria just seems 'elite', while Setsuna has been seen as a grunt child soldier for a weak jihadist movement, suggesting he wasn't in any kind of real program like Soma).