Just a couple of things since I am not too great at WH40Kmyself, but have played guard for 6 years, (and still a noob shame on me).
I do not like the Exterminator tanks. I wanted to use them, they come TL now which is nice. But there are better ways to throw DAkka at people than that Leman Russ.
The Best use i have found for the punisher is to put it with a Executioner(the ones that shoot 3 small plasma blasts) That being for two reasons,
1. The range of the executioner (i think its 48"?) is a lot closer to that of the Punisher than the Vanquisher. Your punisher needs to be danger close, vanquisher doesnt. (72")
Now I dont know what you are using the prim psyker for but i would give him prescience, and give your HQ Twin Linked. It will be..punishing, with your Heavy 20 weapon, and nuts on the heavy 3 plasma blast.
I have seen it first hand, it will bring even SM to their knees.
So If you were to make the HQ change I suggested, I woud drop pask and go with a normal tank commander, as preferred enemy isnt needed if you have TL.
Drop the Exterminators, move the Vanquisher into one of their places.
Ive also never mixed Vets and PCS. So cant say anything about it. But for me their would be one or two more vet squads in chims, and those extra two would have plasma.
Hope I helped or at least gave you some ideas of your own.
Welcome to the Guard Cadet, DISMISSED.