Strength scales at a rate of .45 on tanks, making it equal to vit in that regard. While using strength accs is no longer 'good', melding strength onto accessories remains viable.

As for the rest of your questions, it means you can't overmeld materia into gear beyond the two slots available. On left pieces and weapon, you won't be able to meld vit or str as those are already capped. You can however meld materia th at boosts noncapped secondaries like Parry(why), det or crit.