Welcome and im glad that you enjoy the mod,

As we say against a lot of newcomers' questions is. Read the FAQ!

Romani in the Camilland period can only build barracks in their starting cities + Segesta if I am correct. When the Polybian period starts, the camilland "building" will not change but a new polybian "building" will pop up. If you upgrade your barracks in that period, your new troops will come available. These are polybian type hastati, principes triarii, velites and new Equites. Also you are now able to build barracks accros whole Italy.

~Fluvius