The lava rocks will start having bacteria grow on them as soon as you put them in the water but for a good strong biological filter you need water flowing though them carrying oxygen and nitrogenous waste to the bacteria so they can utililize it.. if you put that lavarock in a cannister or sump or hang on the back filter it would develop bacteria much faster and better.
Your other problem is the established filters with their water and air flowing through it will be breaking down the ammonia and nitrites quickly which leaves less for the bacteria that starts to colonize the lavarock . This is going to slow things down.
What is the reason you want to use lavarock on the bottom of the tank?