In truth I'm REALLY interested in this mech design thing, but so far it's beating the crap out of me. I mean, I get most of the basics, but the Mechtype/number at the top (Buruburus, Vadels and Duams are Battleroid 5, where as the Galah is Battleroid 4.. the Wraith is Aerofighter 4, the Corriach is Aerofighter 6 and the Kraken is Aerofighter 10) is still a bit beyond.. is it a reference to locations?
Th'folks above covered the rest of the questions related to this well, but here's some confirmation on what Beorn said:
Those numbers in particular refer to
size, specifically the size of the mecha in general. A aerofighter 1 will be smaller (weigh less, have less slots for equipment per component, have less armor/HP per body part, etc.) than an aerofighter 10. This is independant of any mass modification or whathaveyou inherent to the limbs themselves.
As for adjusting MV/TR, in GH2 it's accomplished (outside of changing mass) primarily by software. Iirc, it was different in GH1, but I can't recall exactly how at the moment.
To your next set of questions:
1: Give them a faction (Iirc, there's a general one of some sort if you don't want them used by particular groups) and a... role? Something like that, shows up as GROUND, SPACE, etc. Then stick the file in the mecha folder with the rest of 'em and they'll start showing up in shops and combat. Simple as that, iirc.
2: Not really, though it depends on how out of line compared to stock mecha the machine is. Many (most?) winner machines end up buggered with by mecha engineering so much they might as well be homebrew
3: Unfortunately, no. They
can be mucked with, but it takes an annoying amount of effort and, iirc, uses a custom scripting language of some sort or another. It's been done and (again, when the wiki/forum/etc. come back in full) there's some (user created, mostly) documentation for it, but it's a notably more daunting task than stuffing the game full of new items and mecha.
S'the major reason Mr. H and a few others are working on converting GH2 to python (Now... I think it started attempting to convert to LUA scripting, but things change, I guess.), so the rest of the content is more accessible to folks.
E: As for modding weaponry, that's somewhat more involved. It looks like the forum's back up, so go give a gander at the game mechanics subforum, where a lot of the modding guides end up.
This in particular might help, to toot my own horn a bit. I think it was still accurate up until development on the current/old GH2 stopped and the conversion process started. Do note that I probably can't clarify anything in that or later posts anymore, because I don't remember any of it