Should I just leave my GBB replicas at home during this time of year?
I do
Would larger plate carriers mean less need for multiple underlayers (for the sake of the mobility of the arms)?
Plate carriers definately add warmth
What head gear is suggested ("tactical" headgear)?
Usually start with a beanie or fleece cap, then move to a ballcap or boonie, as you warm up. I prefer a helmet tho, a cold ear getting shot sucks
What gloves are suggested (aside from ski gloves)?
Surprisingly... I've liked these the best http://www.westcountygardener.com/glove-waterproof.php
Can snow jam my magazines?
It's a good idea to keep snow out of you magazines and barrel
What camouglage patterns, uniform cuts, boot styles, and gear colors/styles are the most effective/suggested for arctic warfare?
Gortex boots are the best at keeping your feet dry. You'll probably want to get a set of gortex pants. Personally I like winterweight Danish M84 pants and a softshell Jacket. As for best camo for the winter, Pacman can tell you, I think it's ACU/UCP trousers with a Temperate Marpat shirt.
How many extra batteries for my AEG/radio(s)/other electronic equipment should I bring to compensate for the cooling effect on batteries?
Bring one more battery than you usually use