Ryan,
Well aviation reference are best United (and others) understand you’re new and will only have so many. Btw, if you do exceptionally well on a checkride (2+ are better), don’t be afraid to ask the DE for one. Unless you frightened them on the ride then don’t 
Adam
Ryan,
I do not see how you would have too many references available to you at this stage in the game. After you complete your training, you will have more people who are willing to vouch for you.
Honestly though, I think you are focusing on the wrong thing here. You say you have a PPL license, even if UAL will accept an application for their Aviate program, I will bet you $100 that they will not even look at it until you are actually a commercial pilot. I would focus on your flight training and worry about this stuff at a more appropriate time.
Chris