The Mandalorians are a funny lot. They keep getting wiped out to the brink of extinction and then resurface after 30 years. Individually they are superbly talented, but there simply are not enough of them to justify the hassle of adding a faction.
It's a bit like Bond. You can send 007 to assault a single house, commando style, but he can't capture a whole county all by himself. The Mandalorians are the same. Think of them as an elite-of-the-elite covert action group, not an army.
This mod deals with planetary invasions. A single "squad" in PR represents hundreds of troopers, and a "land battle" involves thousands.
By analogy the Mandalorians would probably become a single Major Hero with 3 guards (and a LOT of HitPoints).
In the grand scheme of things there are more important additions to make to the mod.