Actually, I believe that it's possible to make green materia call summons. I know that they can call enemy skills.
However, the materia will not be red any longer. Red materia are strictly associated with one 'type', which is "Do spell [X] the number of times determined by the level".
I believe this to be true, but note that any green (magic) materia will not provide the "summon" menu option.
Say I have made an "Inferno" Materia to have Fire, Fire2, Fire3, and Ifrit on it. I master it and get all the attacks. Now technically I have access to all four of these attacks. BUT, it won't let me perform all these attacks. If it's a green materia, the "magic" command will show up and I'll be able to cast Fire, Fire2, and Fire3, but the "summon" command will not be there unless I have another summon materia on that character.
Making this a summon materia WILL NOT WORK based on the way the game handles summon materia.
Now If I get REALLY wacky and want my Inferno to have "Fire", "Flamethrower", "Fire2", and "Ifrit" the same restrictions apply. No E.Skills unless the menu option exists. Even then, it might not work since the E.Skill menu is handled VERY differently.
This is all theory ATM. I haven't tested any of this. I might do that later today.
i want to do something along the lines of:
AP level 1 = blank
AP level 2 = Bahamut
AP level 3 = Neo Bahamut
AP level 4 = Bahamut ZERO
AP level 5 = MASTER
Just an idea I thought up. I want to remove master materia's from the game, and KoTR (for obvious reason) and maybe is there a semi-easy way to replace KoTR with my new materia (as in: you get the gold chocobo to go the new materia instead of getting KoTR)? I was thinking that maybe I could make it out to be a magic materia in WM, but with summon spells. However I don't know if it would show up as green magic materia or red summon materia, which I'm kinda picky about, but that's just me. Also I was thinking if it's even possible to make this materia that if I just over-write the KoTR materia (also in wallmarket) would that make it so I could in game pick up my newly made materia instead at the area where KoTR would be in the field.
Unfortunately, this is not possible based on how the summon materia are handled. Summon materia only cares about the first modifier slot. All others are ignored completely. Even changing them doesn't give you more summon attacks per battle.