As you mention, some buff can be combined and free up some slot. This would be awesome. As for my self. I find this 6 buff should be combined together, which will free 3 slots of buff.
Buffs, which the 3 buffer class have and should have any conflict are:
Might+Shield (PP, EE, SE)
Focus+Death Whisper (PP, SE)
Blessed the Body+Blessed the Soul (PP)
Buff which one class have but another dont and will have conflict are:
Acumen (PP) +Concentration (PP, EE, SE)
Haste (PP) +Guidance (PP, SE)
Magic Barrier (PP) + Mental Shield (PP, EE, SE)