I put a Plotinus paper in 'Ancient Greek Philosophy, Misc.', which isn't exactly wrong, but may not be optimum.  There's a temporal gap between your last named ancient category (Hellenistic) and the next one (pre-1000 medieval philosophy).  The size of the gap depends on when you want to start the Medieval period.
You've got Augustine in pre-1000 medieval philosophy, which is perfectly standard.  If you want to keep chopping history at the century marks, that suggests perhaps a Roman Imperial Philosophy category of the 1st through 3rd centuries AD.  Relevant philosophers include Philo of Alexandria, Seneca, Epictetus, Marcus Aurelius, Sextus Empiricus, and Plotinus as well as various early church fathers, e.g. Irenaeus, Tertullian, and Origen.  You could still put this under the Ancient Greek Philosophy heading.  I think all of the figures I named except Seneca, Marcus Aurelius, and Tertullian wrote in Greek.  Or you could call it Ancient Philosophy. 

One level down, you could call 'Hellenistic Philosophy' 'Hellenistic and Imperial Philosophy' as an alternative to adding a sparse 'Roman Imperial Philosophy' category.  

You might decide that pagan philosophers, no matter how late, count as Ancient and not Medieval.  Relevant figures are Ammonius and Proclus, 5th century neo-Platonists. Likewise, you might decide that Christians, no matter how early, count as medieval.