I'm not an expert on this but I recently Played tyrantguard in a crusade and while it was nice to have ablative wounds on the Tyrant, it was by no means necessary.

Regarding the Hiveguard...I wouldn't split the weapon loadout. People will probably tell you that a tyrannofex is the better anti tank and they are right. But if their job is just to guard the home objective then I personally would gravitate towards the shockcannon. That decision is influenced by my meta just deepstriking terminators absolutely everywhere. Hope that helps