The Outcast Brigade by Douglas Hirt
They are not wanted where they come from. They are not welcome anywhere. They are outcasts, rootless and friendless, until luck or destiny throw them together. A former Apache scout shunned by his tribe, an ex-Union Army major, a former Confederate captain, and two army deserters, all forced to band together to stay alive Can they bury their anger and work together long enough to do what they have to do? Can they make it to Mexico to rescue a band of friendly Apaches who have been captured and sold into slavery? It is no easy task, because even if they manage not to kill each other, there are plenty of others eager to do it for them.