Yeah, stick to undead Dwarves, the Warlord's guards pledged to serve him in death.

If you really want some Orcs in there, I would go with the opposite approach. One of the rooms in the tomb is a massive trophy room, filled with the shattered weapons and armor of the orcs the Warlord slew. An Orcish Warlock placed a curse on the tomb, now, every night, the spirits of the Orcs rise up, forming spectral bodies that wield their old weapons in an eternal quest to destroy the Warlord.
Each night, the Tomb's undead guardians slay these spirits (Their bodies are perfectly corporeal, and when they die the bodies simply vanish and the weapons drop to the ground). Then, seeking to protect the tomb, the guards simply put the weapons back in the trophy room, causing the whole cycle to repeat.