2nd edition epic is wonderful for blurry rules
For example - chaos
can take any space marnine support cards as support for chaos marine legions - but you dont really know that unless you read a particular Q&A section of an old white dwarf!
Titans were odd too - everyone "knew" imperial forces could take them as special cards - but where does it say that?
Now back to point - Can the Imperator be chosen for "normal" imperial armies, or only titan legion / knight armies? ÂÂ
If it has that little symbol on the card that states its a special card for titan legion / knight armies, then "normal" imperial armies cannot take them - and need a company card of allied tech units to include it.  In your example - the tech guard defense company could NOT be included in the chaos army, because they are tech guard, not technically imperial guard.  (Tech unit cards are more purplish rather than blue and feature the winged skull, rather than imperial eagle).
If this is the case, I would say "NO" for chaos, because it isnt "technically" an imperial guard unit, and thus, cannot be included in the 50% allowance for traitor allies.
If it is just a general titan special card, like individual warlords, reavers, and warhound scout titan squads that ANY imperial army can use, then I would say it's a big YES for inclusion in chaos forces.
I dont have the army cards on me right now (at work), but I'll try to dig up the card tonight...