I also believe that when the AI chooses a system for a reinforcement pulse, it reinforces said system with ships of that mark or the AI tech level, whichever is highest. It's pretty rare to see tech level three (730 AIP) nowadays during a 'normal' game, and most of the galaxy is mark II or above. I believe guardians (due to the fact that they never spawn on the mark one planets that spawn only homeworld adjacent) spawn one mark lower, but at the appropriate power level for that mark.
I don't believe structures such as ion cannons, force fields, and fortresses are tied to system mark. It's more for guardians, fleet ships, and guard posts, iirc.