They are still loyal to the true Imperium though, will fight for the Imperium. It's complicated. But as a result of this, they are made up of soldiers and units from all across the galaxy that fight together.
This sounds like a classic 2E campaign army, composed of surviving bits of various regiments collected together under one banner from the largest remaining force.
I would suggest something like this:
Cadian Command Squad w/ Chimera
Cadian Infantry Platoon (small)
3 Veteran Squads of non-Cadian regiments w/ Chimera
2 Stormtrooper Squads
1 squad Ogryns w/ Chimera
Valkyrie for Stormtrooper
1 non-Cadian Vendetta
Cadian Leman Russ Squadron (2 tanks)
2 non-Cadian specialist Tanks
Unify all of the units with a common Regimental marking, perhaps a big Orange
ID panel.