If you're playing at difficulty <~8, there's time to take a coupleof planets before your assault, for two reasons:
- diffuse AI reinforcements across more planets (depending on how well connected your planets are, generally take ones with 3+ connections as the AI gains more reinforcements for each early planet you take).
- more knowledge to unlock mark-II ships. Having m-II against m-IV is like having m-I against m-III.
And yes, you really want something to take out m-IV guardians, especially laser and zombie guardians.