If you upgraded your MICs (barracks) all the way, then you will have to tear them down and build new ones once you get the Polybian Reform in order to get new units. Marian MICs are placed by script and are a separate building tree, so no worries there.

As far as getting the Marian Reform, 90 settlements is the "OR" requirements. For the AI and in case all of your generals suck. See 2nd post /\