Just to add to what ShakeNBake and Epip said, Erebus is a great choice. Not sure if I would bother with Mist on a minion build. I would use that point on Mass Charm for interrupts. While Cloak of Night is an excellent favor item for most generals (esp. QoT or Sedna), skip it for Erebus in favor of something like BotF. Since Erebus has Bat Swarm, he doesn't need the Cloak. As Shake said, always get monks for generals at the start, even on non-minion builds.
Oak is a solid choice too, even in minion build. I don't play him, but he is a top-tier demigod. I don't play Demon Assassin either, so I can't really comment.
Regulus is tough in the right hands. I always get Snipe for the cheap kills. I would strongly suggest some points in Mines. This gives him some anti-creep, anti-demigod, and flag-control power. I've even seen one guy put mines next to an enemy tower to do some serious damage. Get items that increase Regulus's movement/attack speed and damage output. Poison dagger is a decent favor item for him.