On carapace v flak
Flak is an interesting technology because its a high tech flexible armour (like kevlar) that is designed to dissipate heat from laser weapons and energy from blasts.
It can take almost any form:
Fatigues, overcoat, vests etc. Which is to say, the Highborn regiments are using the same protective tech as the lowborn peasants, even though their uniforms may have more or less frogging and decorative buckles.
There are also heavier and lighter forms of the armour- a Cadian would be wearing heavier flak armour than a catachan's lighter vest (or no vest).
Ingame this makes no difference.
Carapace is a different technology, which is more like traditional armour plate. Carapace is unsuitable for a regular guard regiment because of production costs and mobility concerns.
Its specifically given to storm troopers (tempestus scions) and grenadiers who are not typically called on to stand in a trench for several years or force march across a continent. Storm troopers, grenadiers and by extension, guard regiments with carapace would be specialised tools designed to break hard targets with heavy assault from dropships or careful infiltration.
If you do want a more elite fighting human army in heavier armour I'd recommend scions as a base for the rules. If you just want men of privilege in fancy uniforms then the regular rules should have you sorted and there is a wealth of historical and guard kits that you can use to build them.