In general you Do want a bit of headspace, but you won't get the same krausen effect of fermentation. At 75 degrees you could be done quickly, but its good you held off till the end of fermentation if that's your temp. You can start MLF before fermentation ends, but this is usually done with larger open vat fermentors that could lose their CO2 cap.
What wine are you doing this with? What malolactic culture did you choose to go with?