Would a hose that runs from calipers to hardline on brakes work from the slave cylinder to that hardline? Mine is rotten and I can get a single brake line from rockauto super cheap. They look the same to me. Thanks.

No.. It won't fit the end fittings are differrent.

The slave cylinder ends needs/uses a copper crush washer rather than usual compression/flare brake sealing arrangement. I did use a brake line once, had to shorten the male threads on the slave end so it didn't bottom, and get a generic copper washer to make the seal, but it worked fine. There is an official correct SS clutch line available to do this properly.