I can't answer in game terms since RTW isn't here in the UK yet, but in real world terms the Hastati and Princeps were effectively the same except that the former were the younger men, inexperienced compared with the older veterans who made up the ranks of the Princeps.

The third line of troops in a Manipular Legion were the Triarii, the oldest veterans & armed with spears instead of sword & javelin, so it's the Triarii that you need to build to give yourself an anti-cavalry force.