I redesigned centaurs a while back for fun. The design for my alien "centaurs" is actually ripped from these guys' body plan.

Their hands have two thumbs, and can be used in walking and running just like the other limbs. The smaller centaurs have dexterous toes on all of their limbs, but in the higher weight classes, gripping ability on the back four legs is sacrificed in order to better support the body while standing. The central limbs are set wider than the fore and hind limbs to prevent collision when all six are being used in locomotion.