According to an online English-Tibetan dictionary, Seng is part of the Tibetan word for, you guessed it, Scout. Probably not accurate, or his name could be So Pa (which is another word meaning Scout, amongst other things.)
Yeah, I can imagine Pyro being the most powerful, but the most hard to control and the most contrary. They'd be quite a group all together, wouldn't they? They'd probably have to split the group in two and go on separate journeys simply to avoid detection...
As for Chell, if she was the avatar after scout she'd have to be water. But I think that she would also be good as Air, because she does allot of jumping and running in Portal. And then Wheatley would be Water, GLaDOS would be a Metalbender or Energybender, the Anger, Curiosity, Morality and Intelligence cores would be non-benders, Space core could be an Airbender, Adventure core an Earthbender, Fact core could be Water... or something like that. The only problem is that one, there aren't many characters in portal, and two, they're all robots, which means you'd have to humanize them, which is always dangerous.
THEY'LL GROW BACK! I PROMISE! PROBABLY! KIND OF! IT DEPENDS ON THE MOON AT THE TIME OF THE SOLAR ECLIPSE NEAREST TO WHEN THEY FELL OFF!