As a contracted penetration tester you have a moral obligation to provide the best service you can provide within the bounds of local law and your pre-engagement agreement (in that order).
That said, do your fact checking and get legal advice from legal professionals and make sure to discuss the matter with your employer. They may not be comfortable with you pursuing their employees in this manner. Companies are not always comfortable with social engineering.
Even if you decide not to for whatever reason (legal/moral compass), still discuss the matter with your employer, make sure they know you are limiting the scope of the test to exclude social engineer or these intelligence gathering tactics. It's important they stay informed.