Coming from a union sheetmetal worker, I think it would be safer to use a pair of left hand snipes. Pilot drill in the corner where you are going to take off cuttting. Of course it all depends on how thick the sheetmetal is also. If its 24 ga or higher than snipes would work just fine. They would be alot safer. Clean up the edges if you have any fish hooks with a 90 degree angle grinder.
I would use a sawsall. I would drill pilot holes in the corners where you want your window. My soop isn't metal but that is how I did my window and the openings for my nest boxes. I attached my nest boxes on the out side of my coop with access to collect the eggs. I hope you can post some pics.
Well , we did pilot holes and used tin snips with good results . Have the windows cut but not the door to run yet because the run is not done yet ! Hope to have some pictures soon. Thanks all for your help.