Custom transceiver PCS/PMA interface width?

Thank you very much for taking the time to help me understand all this, this is my first foray into multi-gigabit transceivers.

There is one thing I am still confused on if you don't mind explaining or at least pointing me in the right direction so I can learn about it.

If I wasn't trying to allocate every possible bit (5 frames of 20 parallel bits at 500MHz for a 10GHz serial stream of 100 bits every 10nS), what does the transceiver do if I were only sending 2 frames of 20 bits? How do I choose what frequency to clock the 20 parallel lines into the transceiver at? What does the transmitted signal look like in between frames?

Thank you,

Mark