No do not use metal shims, I've done dozens of 1" scopes in 26mm rings. From memory 5 wraps of cellotape around the scope is all that is needed making sure the last wrap end is more or less 'butted' to the start end if you get my drift i.e. 5 full turns not overlapping. The clear celloptape is best and is available in the same width as the ring. Cellotape compresses to become virtually a solid plastic shim and you get no slippage, not affected by heat and does not mark the scope.
Do not use soft electrical tape as it is affected by heat with the tape 'glue' becoming slimy and the scope will slip in the rings.

Used to open a lot of 1" rings out to accommodate 26mm scopes then reuse them with cellotape shimming when using 1" scopes in these opened out or new 26mm rings Never used anything other than cellotape and never had an issue with scope slippage.