I've got a recently-50-ed Fire/Atomic, and he does pretty well. For me, I skipped the melee attacks in /Atomic, since Fire/'s got plenty of damage already, and focused on the Controls. You definitely want Radioactive Cloud...the only reason to skip it is if you feel it's too cheap too use (seriously, it's a better AoE hold than Controllers or Dominators get), and that power alone really kinda substitutes for most of the defense you can need if you build for sufficient Recharge (which, thanks to Beta Decay, /Atomic is really good at).
As for the Destiny pick...I'm having trouble with it, too, but Clarion's probably the most practical. I don't get mezzed much, but it's kind of a bummer when it happens. Rebirth feels sorta redundant on a Blaster that gets most primary defenses via health/regeneration, and Ageless feels similarly redundant. Barrier might not be too bad, but the longterm advantages of it don't do much to stack, IMO. Still, I haven't completely decided.