In English Pete is a diminutive form of Peter. This means that a person named Peter may permit people who know him well to call him "Pete". American culture being more informal, it is more likely that a man named Peter might introduce himself as "Pete". But he will still be "Peter" on official documents.
Will Rehema be Rahma on official documents, such as her passport, then?
M- is singular, Wa- is plural. Scroll down in this page to see examples: https://www.duolingo.com/skill/sw/intro/tips-and-notes