I don't have FM: Periphery handy, but HB: Major Periphery States says that "The smallest building block in the Legiones Marianes is the century, comprised of...one hundred infantry troopers divided into ten-man squads known as Contubernia." (page 157)
This is fairly straightforward for foot infantry, with a single 100-man platoon being considered a full century(ignoring the fact that in gameplay, this platoon is split up into sub-platoons), but per the table on page 147 of Tech Manual, the Marian Legions use smaller platoons for other motive types, with 50-man jump platoons and 20-man mechinf platoons.
If I'm wanting to stay true to Marian organization, is a single smaller platoon still a century, or should I deploy multiple platoons in order to keep my centuries at a full 100 troopers?