The r-134a sets have sealed hoses that hold gas even after they have been disconnected. Most r-12 sets did not.
For this reason, cross contamination of gases in not likely using an r-12 set on r-134a, though there may be residual ester oil kicking around.
r-134a systems are more sensitive to moisture, so the open hoses are not the best. Really need to pre-purge the air out of them, but the EPA frowns on this (more than a little).
Getting a dedicated set is the best choice. They can cost as little as $75. Good investment, given the cost of A/C repairs.
1986 300E 5-Speed 240k mi.