[Squire]

I'm not a guard player but on paper I like the vanquisher a lot. I think I'd run two in an IG list, probably putting a tank commander or Pask in one of them.

I also like the eradicator and would use it with plasma cannon sponsons, but in your list I think a vanquisher is probably more useful

Also have you considered an imperial knight?

 

[Squire]

If you're getting swept in combat maybe a couple more cheap infantry squads would work as speed bumps. Actually just make them veterans, the extra tax is pretty insignificant from what I recall. Load them with flamers!