I agree with most you say, so I'm not sure why you're
1) When I say, 20000 Persians dies at Thermopylae, that is the only number known to history, and as I note, that's based off of Herodotus.
2) There is some evidence to suggest that the Persian's may have sold prisoners of war as slaves, but as I said earlier, slavery was a crime against Zoroastrianism, and expressly forbidden. Instead, the Persian empire was more like Medieval Europe, relying on local governments called satrapies to hold together the massive empire.
3) As for the army size, 300,000 is probably an over-estimate but most historians think the army numbered from 100,000-400,000. Obviously not all of these men were fighting, but part of the invading army. As for the railroad comment, thats just incorrect. Chandragupta Maurya had the largest army of the ancient world, numbering over 600,000. No railroads then.