There is never, ever, ever a requirement in the law that pay be based on longevity. But there is also never, ever, ever a law that prohibits it. And unless Title VII or related laws are being violated, which does not appear to be the case based on the few facts you have provided, there is never, ever, ever a law that says if some employees are paid based on longevity all of them have to be.
I am speaking from experience when I tell you that after a merger, it can take two years or longer before the wages and benefits get smoothed out. It's typical for employees to remain on the policies of their legacy companies until that happens.