I have always seen my IG Company as being made up of the remnants of other companies and regiments. On the real battlefield you would re consolidate surviving troops into another formation. I also do not see why this could not be a more permanent state of affairs. The regiment could be formed and prove itself battle hardened and simply be shifted onto the next battlefield without breaking them up again and doing a costly amount of moving small squads around.
This at least gives me the excuse to use the models I like rather than be tied down to one particular style of models of type of regiment.
In my IG army I have mostly Cadians and Catachans, but I have a healthy number of Tallarns, old style Stormtroopers, Orlocks, SoB Militia and a few Mordians.
I like to think of them as a platoon of each type. They all have a common painting scheme (although the ones that are more like PDF/Militia/Conscripts have a simpler style and less camo).
In my opinion the older Orlock models make excellent additions to the IG, especially alongside the Catachans. In fact they work quite well the other way round. I always had a Catachan plasma gunner in my Necromunda gang and he did not look out of place.