Combining Mecha tend to combine in a humanoid form. It's easier for people to follow, and it's easier for the guys to get fitting rubber suits. When mecha combine, the parts will more or less match up with limbs, and it doesn't really matter what goes where.
Except when it does.
The "face" and/or torso are special parts of the combined form. They're the most identifiable, and they're generally [...] [tvtropes.org]