It's not breaking any rules.
It's stupid, but legal.
I recall a thread some months ago where white, using your diagam, was taunted into promoting to a Bishop. Black reportedly said something like "I bet you can't mate with two Bishops". As in your diagram, White ended up with two bishops on the same color squares.
I'd promote to a knight just to show off my knowledge of bishop and knight technique.
I know that you wouldn't EVER promote to a bishop or rook normally because you can get a queen which is basiclly both of them added together, BUT if you had one light squared bishop and you promoted to a bishop (noting that you also promoted on a light square) would this be breaking any rules because you had 2 light squared bishops.