I've operated Pa-31's down to about -5C. We used to set throttles full open, mixture rich, pumps on for about 3 seconds (or as required to get about one second of fuel pressure indication), then mixture ICO, throttles cracked til you hear the switch click (about half an inch), mags on, crank until it fires... then rich... (One at a time)
Occasionally after start the engines would struggle to accelerate much past 800-900 rpm with an indicated manifold pressure of 20" which does not respond to throttle movement because the mixture was too rich... So a little bit of mixture leaning smoothed it out and rpm stabilised with a drop in manifold pressure to a more normal <10" approx.
The POH requirement for cowls open on the ground was explained to me as being there to prevent hot spots around engine seals and rubber pipes.