For me the minimum was (rounded):
Sangay - 5C @ 5 AM
Iliniza Norte 0C @ 7 AM (start at 7 AM)
Cotopaxi - 10C (-20 w/ windshield ?) @ 5AM
Chimborazo - 15C (-20 w/ windshield?) @ 5AM
Most people had plastic boots on Cotopaxi and Chimborazo.
I believe a pair of insulated mountaineering leather boots should be enough if you make sure your toes are not compressed (limiting blood circulation) even with a thick pair of sox and after few hours of walk. Furthermore, you may want to keep the boots inside your sleeping bag during the night
As far as me, in order to travel light I chose to go with only one pair of footwear the whole trip, both for cities and summits: opted for my very comfortable Millet Alti (heavy trekking leather boots) rigid enough for crampons, yet flexible enough to run with, but with minor insulation (isotherm 0C according to Millet ). On Chimborazo I took the inner soles out to free room for my 2 thick pairs of sox and added Ecuadorian made not insulated over-boots (kind of Climb High Super Gaiters but with out the “Thinsulate” layer) for wind protection. @ - 15C + wind I reached the limit for this combination.