The most important thing to remember when buying tubing is that the
inside diameter must not be greater than the outside diameter, otherwise
the hole will be on the outside of the pipe. Seriously though, I have
stuck 5/8 ref (1/2 plumbing) copper into 3/4 ref. tubing and soldered
the joint with no problem at all. Also put 7/8 ref (3/4 pl) over 3/4 ref
and done the same thing. It is done all the time in A/C. About a year
ago we had an idiot helper slice through a 1/2" water line(what we call
5/8) with a sawzall. He mangled up about 4-6 " of the pipe. and besides
being nearly impossible to get to (I was the only one who could squeeze
into the space) the pipe would not move at all in any direction. I just
cut a piece of 3/4 ref about 4" longer than the gap in the line, managed
to get it on like a giant repair coupling, and soldered it up. Worked
fine. HOWEVER, I only use silver solder on copper tubing regardless of
it's use, so ymmv using soft solder, but the tubing fits into the larger
one tightly, so there should not really be any/much difference between
that and using a coupling or other fitting. Good luck Larry