"The girls have the latest skirts."
Translation:Les filles ont les dernières jupes.
No, in fact most adjectives follow the noun. Have a read of this article:
While it's true that most adjectives follow the noun, ordinal adjectives like premier, deuxième, dernier, énième... are almost always used before the noun.
The only cases when you could find an ordinal after its noun is in the title of the first chapter of a book, "chapitre premier", and in the title of kings like "François Ier", though with numbers greater than ones, cardinal numbers are used instead, like "Louis XVI" which is 'Louis seize' and not 'Louis seizième'.
I don't really understand your question but I think you are referring to "elision". As you said:
it's 'ont' but not 'sont'
That's because "Ils / elles ont" = "They have"
and "Ils / elles sont" = "They are"
But, in a sentence like "Les filles ont ..." = "The girls have ..."
however it is pronounced, due to elision, *Les fille s-ont...".
I hope that helps ☺
Actually there is no liaison between filles and ont, and even if there were one it would be pronounced differently from les filles sont (for instance 'elles sont' /εl.sõ/ and 'elles ont' /εl.zõ/ are pronounced differently).
And by the way, it's called 'liaison', elision might sound similar but it's pretty much the opposite phenomenon ^^