Should i OTP, have a character pool, play everything ?

Hi ! I want to improve at Overwatch and i was wondering what is the best way ? As a support player i really like Juno and Kiriko. But a friend told me i should have at least 1 single player heal and 1 aoe heal. Is it a good advice ? And if so, is Kiriko Juno and Baptiste a good character pool ? Ty ! (Sorry english is not my first language)