The progression does end at brigands.
They are not mounted, and I found them decent enough soldiers, but, hardly worth the effort to get them.
To see for yourself, what I did was in cheat mode, join a side. Wait untill soldiers of your side attack a bandit, river pirate, or whatever, type unit, and help them so they win. Take none of the prisoners so they do. Then, teleport to an oposing side, pay tribute, and then teleport back and attack them. When you defeat them, you can recruit the river pirates.
This happens naturally in the game as well, while playing normally, I've occasionaly noticed Swadians roaming around with river pirates in their retinue, not as prisoners. My guess is that this occured from killing Veagirs who had river pirate prisoners and then liberating them,