Γεωργός is a bit more "narrow". Αγρότης is any kind of farmer, γεωργός is the one that cultivates the earth, not a stock-farmer. So , οι αγρότες είναι οι γεωργοί και οι κτηνοτρόφοι=Farmers are the agriculturalists and the stock-farmers.
How do you know the proper method to change a masculine word to a feminine word? I know that objects are out of the question, but in Spanish you basically just add an a and sometimes drop the last vowel like jefe to jefa. It seems a bit more complicated in Greek.
Nouns in -τής become -τρια.
Nouns in -ης (not -τής) become -ισσα.
Nouns in -ος stay the same mostly.
Nouns in -ας are very very ruleless and may become -ισσα, -ου, -ω etc.
Keep in mind that the "rules" above can be broken, it's just empirical.