Prototypically, mainlines have the heavier rail (code 83) while the sidings have lighter rail (less than code 83). Code 70 is suitable for a prototypicaly scaled siding.
Down at the model railroad club that I belong to, we try to use Peco switches whenever possible when replacing old rail, or adding new rail. The mainline at the club is code 100 (we didn't build the layout, we bought it from a club that decided to move and start a new layout) Peco doesn't do code 100 (that we know of) so we shim the code 83 switches and rails, bridges, etc up with playing cards up to the correct height, then jam in a code 83 connector, solder the joint, and when the rails are buried in ballast, no one can tell the difference.