Youll definitely want Mana regen and as much as possible.
High Intelligence is needed to aid in Mana regen.
LMC is also paramount.
SDI has no effect but Slayer instruments increase your damage 50% on tribulation, and double your despair damage.
You *Can* have 4 FC / 6 FCR but anything higher than 2/6 a waste of template space in my opinion. Lets say you're a bard/mage. Go 2/6 not 4/6.
I have a few 4x120 bards, on one i have 2/6 and magery, on the another I have mystic/focus, and I run stone form/protection. I have (2 on my gear-2 from stone form)0/6 and its never an issue.
Now if youre some sort of Chiv/bard or SW bard, 4/6 has more merit.