Underestimating the power of employees and customers to make or break you during times of change is insanity. Whether you're a small business with a handful of employees or a giant within your industry, like Wells Fargo and Boeing, the human dynamics are predictable in times of chaos and change. If ignored, employees and customers will get even by walking or sabotaging integration. If they don't walk, they'll slow down, call in sick, sabotage systems or simply become belligerent. Begin by treating this as a blend of two families through marriage.
Size is irrelevant.
In the case of Wells Fargo and First Interstate, Wells Fargo neglected their new employees and new customers. They wrongly assumed that virtually all stakeholders would welcome this marriage, despite the fact it was an unfriendly merger. The stakeholders got even for being ignored and discounted in the process, not unlike kids determined to get back to the status quo by destroying the love between the new couple. Avoid the consequences of anticipated profits and growth going south. Do your due diligence before the union is consummated.