So this is a fluffed-army I put together after getting a box of complete mismatch IG parts. This army includes infantry, heavy weapons, and vehicles from Cadians, Catachans, Armageddon Steel Legion, and Valhallan Ice Warriors. I had all of these random pieces and couldn't figure out how to put them together. I also bought Elysians to further the fluff.
I build countless army lists depending on what type of player I'm facing, I've got about 8 different configurations that I keep on paper, but I'm always changing them. This is simply a basic outline for anyone wishing to use an army similar to this, or if they find themselves with a bunch of mismatched pieces as I did.
This army consists of the finest Veterans from across the Imperium. Because of this, regular Infantry platoons aren't even used, instead replaced by Veteran Squads. It sounds limiting but the CACF is known to employ Penal Legion Troopers alongside their Veterans. Anyone that has read the Last Chancers books knows that some Penal Legions are used in special operations as well as meat-grinder operations. I employ transports for most of these units, except for those including heavy weapons and/or snipers.
In terms of Elites, all are great choices. I typically go with Ogryns lead by Commissar Yarrick, Sly Marbo, and either Ratlings or the Psyker Battle Squad loaded in a Chimera.
I use Valkyries loaded with Elysian drop troops which I play as veterans, I also use my Carapace Armoured Veterans in the Valkyrie, I've painted them like Harkoni Warhawks. From time to time I use Rough Riders from Atilla, but mostly I use Cadian Armoured Sentinels or Catachan Scout Sentinels which outflank.
As far as heavies go, do what you wish. I normally play with two Leman Russ Squads of two each, and either two Medusa's or one Basilisk. I hope to get a Manticore soon.
I can post any of my army lists if anybody would like. I have lists for: anti-armor, anti-personnel, objective capture & hold, defensive, blitzkrieg, hero-based army using the plethora of special characters now available, and a meat-grinder army.