There is nothing wrong with using the copper pipe on the smaller diameters but you need to be careful not to allow things to rub on it, its best to clip it down with rubber lined p clips. Also the plumbers merchants can supply 15mm and 22mm pipe in chrome plated copper and in polished stainless but you will need to form an end on it to stop the pipe walking off the end. So heres a locost/tightwad method I use .
We are on plumbing size pipes so compression olives are available for pennies.
Using an active flux solder an olive on the pipe.
That gives you a home made bead on the pipe.
On to exhausts.
Bob