Is it reasonable to ask candidates to create a profile on Google Scholar? I was in the evaluation committee for the adjunct lecturers this year, and it was a complete nightmare. There were over thirty candidates and many of them didn't have an account on Google Scholar, which made tracking their impact really hard, due to name collisions etc. 

My question is would it be a reasonable requirement to force the candidates to create a Google Scholar account, so that we can easily track their publication/citation record and impact? 

My hesitation is that it would require them to give away private information to a third-party company, and some people wouldn't like to be forced to do it, or even raise legal issues.
As much as I like Google Scholar, requiring candidates to create a Google Scholar profile specifically seems inappropriate. You are effectively saying you won't hire people that don't use Google.

What you could do is make it an optional part of the application or you could ask candidates to submit something more vague like a "citation report" and suggest that a printout of their Google Scholar profile is sufficient for this.