We say 'τριακόσια' for 300, 'τετρακόσιες (χιλιάδες)' for 400.000.what is the correct option, τριακόσια or τετρακόσιες?
That's because χιλιάδες is grammatically a feminine noun (singular χιλιάδα, cf. εκατοντάδα = a hundred, δωδεκάδα = a dozen etc.), and numerals for 200, 300... 900 are inflected for gender and case.We say 'τριακόσια' for 300, 'τετρακόσιες (χιλιάδες)' for 400.000.
"Εκατόν" is used for numbers between 100 and 200.εκατον - εκατό
Are these two forms used at random?
I don't agree with the second sentence. In my speech at least, the "-ν" is always pronounced in εκατόν δύο, εκατόν δέκα, εκατόν σαράντα, and before π and τ it usually voices the consonant (in other words, I pronounce either εκατόν τρία, εκατόν πεντήντα as written, or else I say εκατόν dρία, εκατόμ bενήντα.) I don't think it is ever omitted without leaving a trace."Εκατόν" is used for numbers between 100 and 200.
When speaking, that extra "-ν" isn't that convenient, so it's usually not pronounced (when the next number starts with a consonant):