You should only need one Gaea's Ring, as Diablos and Brothers (I think) learn HP+80% themselves. Use an Amnesia Greens on one of the GFs of the other character and use the Gaea Ring on that GF. Remove a junction skill that another of that character's GFs already has, such as Mag-J (quite a few carry this one). SumMag+10% is also a good one to take out on a GF you don't use often.

To junction status magic to your attack/defense, you must have a GF equipped that has the Status Atk-J/Status Def-J skills learned. To junction more than one magic to defense, you need to have Status Def-Jx2 or Status Def-Jx4. Same goes for elemental junctioning.