What happens if a slug is already taken?

GoloLink tells you immediately and suggests alternatives.

Slugs are unique per domain - the same slug can exist once on golo.to and separately again on each of your own custom domains. If the slug you typed is already taken on the domain you picked, GoloLink flags it as you type and suggests a few close alternatives. Pick one of those, try a different domain, or edit your own slug until it's free.