I don't know how many gems you must apply to guarantee that your script will work. I wonder that myself... it seems that I can usually guess right. But mages can certainly use loads of extra gems, as long as they don't use more than one per magic skill level per casting. This thread hits that question about halfway through:
http://forum.shrapnelgames.com/showthread.php?t=45210
Particularly, I often wonder whether the AI makes any effort to save gems that'll be needed by a later part of the script. I suspect it doesn't... maybe somebody who looks at debugging logs can give some evidence?
(I can't address the communion part of the question.)