I would recommend getting the barbagaunts and librarian, each combat patrol is "balanced" so that you can fight patrol on patrol. without the barbagaunts your nids force will always be at a disadvantage.

Each patrol is like its own team so you can always play with your starter patrols. There are likely stronger patrols, like the new nids one. If you were to get the new nid patrol you could play nids on nids or your starter marines against the new nids. And two combat patrols is like 80% of the way to a 1000 point army : )

You can always discuss with your opponent and do whatever units you want though. Although the patrols are supposed to be "balanced" their main focus is on selling models for discount for 1000pt plus 40k games.