Narrowing the body and sleeves

I'm going to make a PJ top from this pattern - right now my son fits well into the 18-24mo size as a shirt, but it's a little roomier than his RTW PJ tops. I was thinking I'd try to match the fit of my favorite of his current pajamas, which would involve making the body of the shirt narrower by about 3/4 in (the sleeves are also more close-fitting). However, the shoulder of this particular top is as wide as the 18-24mo School Bus tee. So I don't want to do a straight size down. Can I use the 18-24mo shoulder width, but then narrow the sides of the shirt under the armscye to the 12-18mo width, and then use the 12-18mo armscye and sleeves?