Phalanx does make sense as a defensive cooldown/spell for RDM (since the whole idea of a phalanx is a defensive formation that is difficult to penetrate). Not to mention the likelyhood of PLD getting a passive reactive ability is rather low as revenge-type abilities are designed around player input rather than activating on their own independent of other actions/animations.
@OP:
I can see the intent behind the system, since you're treating melee skills are a sort of resource builder and make spells your resource spender depending on what combo you used to determine how many resources are spent.
Thematically I don't think ACN would be a good cross-class for RDM (I assume you're doing it for Eye for an Eye and Virus). Something else I wanted to mention is that in order for your spell mode buffs to work, you'd need to make your spells require them to be active. The base forms you've listed are sort of redundant (Inferno and Radial Magic), and if you're trying to go in the direction of modes you need to give each spell three different forms (one for Radial Magic, one for Decaying Magic and one for Charged Magic). Of course, you then have the risk of having spells that do similar things with different names (the additional effect from Decaying Magic).
I'd recommend cutting your elements down to three just to give yourself some breathing room design-wise. It'd even make sense since our THM/BLM is limited to three offensive elements, and RDM here like in the prior FFs should be borrowing from it to a degree.
Assuming you were to cut it down to Inferno/Cocytus/Levin:
Inferno: Requires Radial Magic, Decaying Magic or Charged Magic
--Radial Magic - Deals fire aspected damage with a potency of 100 to target and nearby enemies.
--Decaying Magic - Reduces target's damage resistance by 10% for 10s.
--Charged Magic - Deals fire aspected damage with a potency of 280.
Cocytus: Requires Radial Magic, Decaying Magic or Charged Magic
--Radial Magic - Deals ice aspected damage with a potency of 30 to target and nearby enemies. Additional effect: Slow for 15s.
--Decaying Magic - Binds target in place for 10s. Damage taken breaks the effect.
--Charged Magic - Afflicts target with Deep Freeze, preventing them from moving or acting for 20s. Should the frozen target receive damage from any source, target takes 10% additional damage from that attack and cancels the Deep Freeze effect.
Levin: Requires Radial Magic, Decaying Magic or Charged Magic
--Radial Magic - Deals lightning aspected damage with a potency of 30 to target and nearby enemies. Additional effect: Lightning damage over time with a potency of 30 for 18s.
--Decaying Magic - Deals lightning aspected damage with a potency of 50. Additional effect: Stun for 5s.
--Charged Magic - Afflicts target with Seizure, causing them to take damage every time they perform an action (auto-attacks and special attacks) for 12s.