TT Double Rules

Scenario:

A&B will play X&Y in doubles

A serves to X

Next game X&Y team is to serve...

Question: which one is required to serve X or Y or can they choose who serves?

 (In all games afterwards I understand A & X will serve or receive to each other and the same for B & Y)

Question: Are all double team members  (A & B & X & Y) required to serve in a match of seven games?

Question:  When its A&B time to serve throughout the match, can this team always choose to let A serve and therefore the match can go without  B ever serving?

I always played this routine in the serve order:

A to X, X to A, B to Y, Y to B and then rotation starts all over again.....not true?