Is it even worth worrying about hybrid pieces when it's not impossible to fit PDT/MDT/TP/WS/Idle/Mage-Nuke/Mage-Enfeebling/HMP sets with 9 Staves and 2 swords within the 80 slot limit? =/
The easiest way to condense your gear needs down is to get better gear, as it means you'll need fewer pieces to reach the same/higher stats. I don't ever bother melee'ing with my RDM personally, but I took a few minutes to try and come up with an example Inventory that fit pretty much everything you'd need in it. It's not the absolute ideal, but it's extremely solid and sits at 79/80 Inventory with Shihei included.
Code:
Almace
DA+ Khanda
+2 Water staff
+2 Ice staff
+2 Wind staff
+2 Light staff
+2 Fire staff
+2 Thunder staff
Terra's staff
+2 Earth Staff
Pluto's Staff
Vallus Grip
Raptor Strap +1
Tiphia Sting
Witchstone
Duelist's Chapeau
Warlock's Chapeau +1
Walahra Turban
Estoqueur's Chappel +2
Darksteel Cap +1
Maat's Cap
Artemis' Medal
Orochi Nodowa
WS Gorget
Twilight Torque
Novio Earring
Suppanomimi
Hecate's Earring
Brutal Earring
Loquacious Earring
Antivenom Earring
Merman's Earring
Warlock's Tabard +1
Duelist's Tabard
Goliard Saio
Augur's Jaseran
Estoqueur's Sayon +2
Teal Saio
Coral Scale Mail +1
Antares Harness
Darksteel Harness +1
Dusk Gloves +1
Estoqueur's Gantherots +2
Serpentes Cuffs
Eradico Mitts
Darksteel Mittens +1
Warlock Gloves +1
Dark Ring AUG 6/6
Dark Ring AUG 6/6
INT+ Ring
INT+ Ring
MND+ Ring
MND+ Ring
Minerva's Ring
Rajas Ring
Hoard Ring
DEX+ Ring
Atheling Mantle
Lamia Mantle +1
Umbra Cape
Aslan Cape
Gleeman's Cape
Goading Belt
Witch Sash
Cuchulain's Belt
Sagacity Lappas
Estoqueuer's Fuseau +2
Nashira Seraweels
Tumbler Trunks
Crimson Cuisses
Darksteel Subligar +1
Refresh Subligar
Dusk Ledelsens +1
Estoqueur's Houseaux +2
Duelist's Boots
Serpentes Sabots
Darksteel Leggings +1
Lithe Boots
Shihei
A lot of jobs can do a lot of things, not just RDM. Trying to advocate god-mode Hybrid gear is just silly when the job is already incredibly powerful. As with any job, when you go out to do something you generally have to pick and choose what gear sets you bring based on what you will be doing (easiest example, old-school PLDs had to put away a lot of other gear in order to carry Fire/Earth/Ice Kits for applicable fights). If you want to do everything at the exact same time, well, yes you're going to run into inventory issues. That's not a problem limited to RDM, and it's not one that it's at all possible for the game to fix (PS2 RAM limitations).
Either way, RDM doesn't "need" 120 slots to have melee gear and acceptable mage gear. Personally, I think it's folly for RDMs to stock up on 80 slots worth of melee gear and completely ignore, or use completely sub-par casting gear to make up for it. RDM can do a lot of things well, and yes one of them is Melee, but it can be a massive inconvenience to the group when a Mage class refuses to cast spells. It's important to remember that melee is an addition to Enfeebling, Buffing, Healing, and Nuking; not a replacement.