Welcome to Librarium Online!
*see the killteam vs killteam thread for what this is all about*
first of all warbands-
Orks- done (see Gorkamorka)
Imperial Guard- done (see Necromunda)
Space Marines- this is the major issue here, how realistic do we make Marines? i'm considering giving them something to help with ammo roles in order to make them seem more professional. skills along the lines of true grit would help to represent Space Marine recovery rates.
Necrons- how do we represent the "we'll be back roll" along with the Necromunda recovery roll?
Tau- what maximum stats should the little smurfs have?
Tyranids- here i'm at a total loss
Sisters of Battle- how will acts of faith work? or would Sacred Rites be more apropriate?
I still hate Twiglets!
I don't think most of these armies would end up in skirmish type battles, necromunda made sense they were gangs that never had huge numbers, these groups have huge organized armies and the chances of a kill team encounter is low ( a small force against a large one) but the changes of 2 small forces is really low. the best i could think of is if you looked at the ashen wastes rules found here: necromunda reference and more specifically here: ashen wastes (click the links list on the right side its kinda small and obscure it has the list of rules, gangs, ect.)
if you had a planet that say caused spaceships to crash on it, you would end up with several survivors that would form into tribes of what ever spiecies (pretty much anyone) had crashlanded. Then all the ammo rolls would make sense as they have no supply lines and such. and i don't think the other races would transfer very easily to necromunda, most orcs have the stats of seasoned veterans in necromunda, so i'm not sure how to convert it.