Sorry, arming coat was my suggestion since I have one, it has worked well the last 3 years, will be totally hidden by the plate, and with the caveat that the kit isn't specifically for living history standards. My first suggestion was to make a Charles de Blois like Sir Ian did; arming coat was second. With multiple layers - undergarments, arming clothing, maille, and plate, less bulk seems to be better from my experience.
I've done a haubergeon under my plate before, and with a linen shirt, the double-layer arming coat, haubergeon, then full cuirass, it really isn't too bad. The flexibility at the elbow is the main thing for me; too much bulk, you can't bend or move right. Gambesons with the padded arms hinder too much; Chris and Brian (my brother) both had arm bending issues in gambesons with 14th century style armor; wasn't made for their size specifically, but bulk was the reason, you could see it bunching when they bent their arms.
I don't even like 9mm maille for that very reason, and I'm having heck with my voiders because of it. If I made a fully custom garment, I'd go with no padding on the arms except the wrist/cuff, back half of the elbow, a double-layer on the shoulders (shoulder fatigue reduction), and none in the torso except for a single layer on the hips. I'd consider it my "luxury" arming gear.