Use your best judgment on who you decide to ask - there is no set formula for who should be your recommenders. We know it is not always possible to have a direct supervisor write your recommendation – we would not want you to jeopardize your current position for the application process. Look at the questions we are asking recommenders to complete. Find people who know you well enough to answer them. This can be a former supervisor, a colleague, someone you collaborate on an activity outside of work. How well a person knows you should take priority over level of seniority or HBS alumni status.
Overall, this is an excellent start to a compelling essay. Keep in mind that although the committee allows you to write up to five pages, three double-spaced pages is usually adequate. Feel free to add more detail, but make sure your text is concise and transparent. You might consider "writing" ideas and then "rewriting" them. Too often, authors put ideas onto the page, but do not render those ideas compellingly. Giving your sentences a bit of extra effort, and giving your experiences a bit of extra reflection, will result in a persuasive essay that compliments your accomplishments and character.