In theory Camillan units are available in central Italy, Polybians in all of italy in most marian units in all romanized provinces, before that they aren't much different recruitment wise than the the other govs, but you can build more roman buildings and get more money from them than allied govs. Recruitment areas for roman units should be part of their description.

In 0.8 the whole roman recruitment is somewhat buggy, only in 0.81 it will work as described above.