Beauty rings and valve stem length

[purple gas]

If you take the rim and the ring to the shop they will be able to determine the length of stem.
Unless they're 100% incompetent ( not impossible, I used to work in the tire shop supply & equipment business) there should be no way they can ruin your rings.
Anytime I've replaced tires I get new stems, stems can fail, especially if they're old.
If you get the proper length stems you don't need extensions, they can also fai, leak, or fall right off.
It'll cost a few extra bucks to do it right, but a flat or leaky tire can ruin your day,