I just posted this in another thread:

The Classic Navy Pea Coat...
I've worn a Navy pea coat with the kilt and been pretty happy with the way it fits etc, and it's super warm of course. You won't need any other layers with the pea coat! Mine is basically a military issue jacket and so dark navy that it's almost black. I think it should work well with virtually any tartan. It's also got this great big collar that reaches almost to your ears when pulled up! Nice against the wind, snow and rain.

I think that pea coat style is kinda comin' back in now, too. Check with some of the bigger department stores to see if they've a stock pea coat that's maybe lighter than a regulation wool one. Ya know, I keep tellin' folks I'm a trend setter, but they just roll their eyes at me... Goes to show ya that if you hold on to your clothes long enough (say twenty years or so) you'll be right back in fashion again, no bother.
There really is no need to cut down a pea coat for a kilt simply because IMO it is not a fitted garment (like a sport coat). I can't imagine it would be anything but ruined if you tired to cut down a regulation Navy issue pea coat.