Not necessarily. Scaly leg mites live on the bird full time and don't survive long off the host so they don't spread as fast and easily as other mites and lice. It won't hurt to cover all the birds legs in vaseline, but generally I only treat birds that have raised scales/crust. It would be a good idea to check and/or treat everyone for body mites and lice which do spread quickly.
A month-long quarantine is always advisable for bringing in new birds. Scaly leg mites aren't a big problem imo and I wouldn't be worried about that, but there are other diseases, viruses, etc that the owner may be unaware of that could affect your birds. If they've ever had a respiratory illness they are likely still carriers even if they don't look sick. They stress of being relocated and adding new birds to flock reduces the birds' immune systems so the new and old flocks are more susceptible to falling ill from both old and new pathogens. By keeping the new birds quarantined you can see if any old illnesses pop up before introducing them to your flock, though if you want to be 100% sure they can't infect your flock you need to be quite strict with biosecurity practices (keeping the birds distanced, changing boots and clothes between the groups, etc) which not everyone has the set up or time to do.
