This is a formulaic spell, can it be cast for use on another formulaic spell or must the spell it assists have to be a ritual. I have it in my mind that it must be a ritual but now I look I can't find anything to confirm that.
If it is cast on a formulaic spell (assuming it can be) does it have to be fast cast as the muto vim rules see to suggest.
Can it be cast on spontaneous magic? Would this also need to be fast cast? How about if it were ceremonial spont magic?
Yes, but there is some Errata. Wizard's Communion affecting ritual spells needs to be Sun duration, so it takes two magnitudes of power away from the communion calculation.
Fast casting is a possibility, but you can also hand wave it as the next spell the person leading the communion casts. There may be reasons to fast cast it. I think the errata talks about this a bit.
Muto Vim spells don't work on spontaneous effects. The insert for Muto Vim states that explicitly.