I'm just a novice and I don't actual play
40K, preferring the painting and lore side of it, but, not to sound too boastful, Ancient Greek History is a particular area of interest for me so I may be able to help.
I think your main difficulty will be with how your army is going to perform:
First off, not that I claim to know much about Tau armies, I gather they are notoriously 'shooty' - this is going to be in direct conflict with your Greek theme, as the Greeks were famed for being close-quarters phalanx fighters. It wasn't until the Pelopennesion War (Athens v Sparta), in which Iphcratid reforms of the Athenian General Iphicrates produced a distinct shift in Greek combat practice to include large numbers of peltasts within armies. This is going to be your main problem, but also the battle-suits and vehicles of the Tau are going to be effectively serving as modern cavalry - again, not something the Greeks had lots of. You may want to focus on one of 2 main areas of Ancient Greek History - Classical Greece (Greek City States, Greco-Persian Wars and Pelopennesian War) or Macedonian and Hellenistic Greece (Alexander the Great and the Diadochoi Wars - the wars of his generals after his death).
However, all is not lost!:
- I reckon by giving your Tau Firewarriors a combination of round shields (Possibly some sort of hand-held shield drone), and a pulse-rifle (maybe with bayonet) in the form of a long spear you'll get a distinct 'Hoplite' style to your Firewarriors.
- If you choose to include Kroot they're slightly barbaric look and tendency for light equipment will tie in perfectly as the peltasts of Ancient Greece.
- Your vehicles are less of a problem as obviously you want to retain a Tau army, so people will recognise your deviation from the Greek theme in that respect - Whilst Ancient Greek combat was not centered around Cavalry, they certainly did make use of it (Especially in the Macedonian Era), and there were city-states of Greece such as Corinth, Thessalonica and Apollonia that were famed for their cavalry.
-Obviously, if you go for the Classical Ancient Greece period as your theme, you can include all the horsehair crests you want on your warriors!
Hope this helps (Despite it's length), and if you have any more questions feel free to ask!