My tennis serve question?

Yes, this is legal. It is a good idea to be over the baseline AFTER your serve so that you can be ready for the return. Also, it means you are putting your body into the serve which is very effective. I believe what your worried about is a foot fault- a fault where the server's foot is placed within the court or on the wrong half of the baseline, prior to the ball being hit, resulting in a foul serve.