Q. Should I apply for a white-list account before I start coding?
A No. White listed accounts are currents not available. However, you will find that having 350 calls per hour is plenty as you learn how to develop your program.
Q. I plan to make a simple Twitter application now, but I may expand it later. Should I bother setting up a separate twitterAPI class?
A Yes. If you have any plans, even just thoughts of doing something beyond a few different types of API calls, set up a separate class for your API calls. In addition to new APIs, current API calls can change.