Considering it's an instance that's quite a hard problem you ever thought of just making multiple emitters with different sized bubbles that would ease things up a bit.

Or maybe altering the emitter using mel and just telling it what you want to emit using a rand() function for size on your bubble object.

How much mel do you know it might be easier making a loop that creates sphere geometry with a random size then assigns your shader or texture then assigns any physics to it.

Actually that would probably be what you want to do even though it sounds like a pain in the arse.

Adam