FFVII Limit Breaks, Submarine, Materia, etc

Okies, here is a bit of a barrage of questions, so here goes:

I am intending to have Limit breaks work like FFVIII, so when you have the "Near Fatal" status you can use them, how would I go about implementing this?

Also, in FFVII you had a submarine you could dive with; would it be possible to use the WoR as the undersea regions, say have a certain button change you from the Wob (surface) and the WoR (underwater)?

Regarding materia, in VII you had 3 pieces of equipment, Weapon, Armor, and Accessory, in VI you have R-Hand, L-hand, Head, and Body plus up to 2 relics. I am intending to reduce that to Weapon, Armor, and 2 materia slots, plus 2 accessories. A full implementation of VII's materia system will likely be impossible, but I would like having a red materia add the summon command, and green the magic command, etc. I know certain gear can add/change your battle commands so it should be doable. I was thinking you learn magic from the materia, but can only use it if its equipped, say learn cure 1 from the heal materia, but only be able to use it when a green materia, regardless which, is equipped.