hired and laid off in a merger

What is the name of your state? NY

Is it legal for a company to hire a person with a merger about to happen and then have the person laid off after a few months on the job?

Yes. If the employer knew the job would only be short-term (which may well not have been the case) then it was a lousy deal but absent an employment contract, no jobs come with a guarantee as to how long they'll last.