Okay some comment on the Arch Commandos.
I don't really think I agree with the idea of a test subject being passed around between the clans... that seems too cooperative for the Skaven. lol.
Maybe if you change the fluff around where a particularly independent Skaven hero feels the urge to exploit the different major clans as a sort of thief character, it might sound a little Skaven, whilst, remaining out of character - since all Skaven fear and have some kind of respect for the greater clans, but this guy... just doesn't.
Having said, I think it's better if you stick with a single super hero rather than a duo with a special regiment. A sort of... DOW general/assassin.
There seems to be too much equipment at their disposal ... and the idea of the greater clans handing them out seems ... un-Skaven. And for them to be notorious thief (or thieves if you stick with a unit) then it seems odd that they would haul such heavy and heavy maintenance weapons like pistols and ratling guns.
I don't know, I just don't like the unit as is, and maybe that's because I simply have different perception of what I want ... lol
sj