Never late! Thank you! ;)

Some of the units are only available after a certain turn, with the Gaius Marius Army Reform (which was a historical event, it reformed stuff like soldiers salary, they got more professional as an army and basically the armor changed to what you visualize about the legionary figure when it comes to your mind - like, they left the hastati, principes and triarii to the legionary that you usually see in movies, that look like some anti riot cop outfit).

I belive they are only a mercenary unit. Not always mercenary units shows in the roster of certain factions when you're doing custom battles (this doesn't mean you can't hire them in the campaign mode).

Roman archers are units available just in certain DLC's, depending on the historical period the DLC focus. Roman started to use archers in the time of Augustus, 1st century CE. They incorporated some horse archers as well due the contact they had with wars with the Parthians and saw the potential of it. You unlock roman units as you develop certain technologies and some just come a bit later on the campaign, like the ones from the Marius Reform.