Cent is invariable when it comes before
mille, even when it it multiplied:
deux-cent-mille
Million and milliard are not numeral determiners but common nouns.
So they agree and are written:
deux-cents millions
de poulets
or :quatre-vingts milliards
de fourmis.
In titles of rulers, cardinal numbers are used:
Louis VI le Gros (Louis six the Fat)
Louis XV le Bien-aimé (Louis quinze the beloved)
except for 1 that requires the ordinal number premier:
François Ier is pronounced François premier.