I use three lines, so there are no genuine gaps to be filled. I also sent the Principes one by one into the battle - just where there are needed, what must not be the part of the fighting right ahead of them. I keep the Triarii in reserve until either my line tends to break even with the Principes involved, or the enemy cavalry is somewhere appearing en masse and spearmen are desperatly needed.
That is for the Polybians. In Camillan times I use some classical hoplites tactics. That is the Triarii (here the same number as Principes or Hastati) in the center and the Hastati and the Principes on the wings of that. The Romans had allready abonded this formation in 272 BC for a good century, but I don't want to play two Manipular periods but want to experince the change from Hoplites to Maniples.