Any ship I add, including Tactical Destroyers, shows low/med/high but nothing except for rig slots show when adding a fresh Strategic Cruiser fit. Import for T3 cruisers still does not work either. Any ideas?

You have to add subsystems to get your slots
Yeah, slots are determined by the subsystems you have installed. So go ahead and add those and you should be good to go ;)
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You have to add subsystems to get your slots