Good news for you: The current and previous Guards all use the same sized bases, so you're fine in that regard. Same for the old
HT, so you're still good for the choice you want to use since the Tyranid Prime has never been stated to be any larger on a base than the standard Warriors.
As for using the Tyrants as guards, I'm all for it.

I'd say save em for the Swarmlord, just to further show his status as the Tyrant of Tyrants (plus I'm all in favor of having variations in units... right now working on getting all my termagant heads to be different per unit so I can pick them out).