One thing to bear in mind is that you over engineer, but often everything else is over engineered as well. My 4HP Hurco mill actually has a 5HP motor on it. It came that way. I figured I needed a minimum 5HP (after derating VFD) but it had a 3.7Kw 3ph derated to about 2.4Kw (3HP~ish) since I am feeding it single phase. I didn't have the cash for the VFD I wanted at the time so I just ran what I had. Its been that way for many years now, and I use that mill all the time for aluminum, 4140, 4140HT, 1018, mystery metal, brass or whatever else needs cutting. I don't have to put a 5HP load on it just because it has a 5HP motor. In all my calculations I put 1.5 down as my max spec, and it removes metal at incredible rates. Sure I could cut at 3HP loads still, but its hard on that old 1981 Hurco mill.
Anyway. Yes a 15HP RPC might be a little light if he was running all his motors at peak load, but in reality that just isn't going to happen. The worst current will be inrush, but we aren't talking about a universal motor. We are talking about an induction motor. Its sure not going to be 10hp 7.5kw to start. Then how would I know. Maybe he plans to do high feed high DOC hard steel all day long and throw chips big enough to take out small animals in their path. LOL.
If you want another example. Go look at your home service entrance. Its very likely if you add up all the breakers they exceed the current of the main breaker. How often do you trip your main breaker?
Now we are talking about shop equipment. If you are running all your equipment at the same time, and it all starts a hard cut, hard start up, or crash at the same time... Then you will be lucky if your main trips right away. LOL.
Anyway, I would agree in general to go bigger, but as long as he isn't running actual 10HP loads I wouldn't worry to much about it for occasional use.