I had no knowledge of Alpha Legion before I read it. It was confusing at first but I started understanding it and it ended up being a great book. It will start making sense after a while.
The Geno's are not a race, they are a regiment of the Imperial Army, Bronzi and Soneka are the two main guys in the story other than John, their captains of the Geno's. The enemy is the Nurthene, they are human, like a few of the other worlds in previous books. And John is human, but he isn't Imperial or Nurthene, hes trying to find a way to get inside the Alpha legion and talk to their leader.
Hopefully that made some more sense and didn't really spoil anything, just keep reading, it does get really good.