Though we have seen the Centaur Mystic of the Sisterhood, here are the three neutral Centaur units that will make up your shortcomings.
The Rainstriders are the only Centaur race that are powerful, but very peaceful. They made a pact with the Sisters of the Empire due to an eerie relations and friendship with these female horsemen.
The Sentinel is a medium ranged unit that fires Cold Arrows to slow down enemy's attack speed and movement.
The Shaman is a less aggressive, but very instinctive version of the Centaur Sorcerer that hurls storm bolts and casts the same abilities like the Sorcerer.
Lastly, the Rainstrider Queen is a powerful heavy ranged unit and a relative to the Centaur Khan, but has a Trueshot Aura to increase the damage output to ranged Rainstrider Centaur units that will even the odds.