1. If you are referring to a run-of-the-mill company, the words company, enterprise, and firm are often quite interchangeable. However, I would not say that they are *always* interchangeable. For example, you cannot say "law company" for "law firm." I would also say that an "enterprise" sounds larger than a company, which has a secondary meaning (an assembly of individuals for social purposes). "Enterprise" can also connote a spirit of adventure (related to the adjective "enterprising"). Sometimes using one word also just sounds better than using any of the other two. The good thing, however, is that no one will misunderstand you if you use one word for any of the other two.
2. It is called a merger or a corporate merger in English.