Good idea to try it out on sacrificial trousers! Much less anxiety that way.

I learned about gussets from a book on sewing handwoven fabric into garments using only rectangles, as it's a way to add ease without using much fabric, and with no waste - always a major goal with handwovens. Here's a coat where I used an underarm gusset. The sleeve portion is at the top of the picture; the garment side seam at bottom center. It worked like a charm - even though the sleeves are fairly narrow, the coat is comfortable and with plenty of easy of movement.