TBH..N/A. You should resell those items if you can.

Heavy Darksteel Armor / Flanchard are useless. They take up 2 slots each (Head/Chest & Legs/Boots), but still only allow 5 materia slots. You would need to be able to meld up to 10 for them to be remotely useful.

Save your Gil on Materia and just re-sell the items.