Can't go to 0, the reason why classes under 30 can't desynthesis is because their skill level will be 0, need at least 1 to do it and they won't let you lock yourself out of certain class desynthesis.
You can get three to 100 if the other 5 classes are left under 30, but I really doubt those 5 missing points will matter that much.
I'm thinking about getting ALC to 50 just to be able to get my own shiny weapons, the other two I dunno yet, probably WVR and something else.