Because he doesn't kill. The only dragon ever said not to get along with the others is Setsuna. Madoka is kind, and so is Koku. They get along so they'd be compatible. Still Koku himself is deadly. Madoka would be just so they couldn't dodge the beam. I'm assuming Kyo is fire proof anyway that's why I didn't say Setsuna. Still he wouldn't have to combine Rui and Koku and still achieve the same effect. Create an illusion and then use the laser..or Homura which gives him super strength.
I can't remember if Kurei's souls do anything at all besides change the shape of his fire...I'm actually wondering how Kurei would do. He doesn't really have the sure fire kill Recca does, and I'm also assume Iori is as fire proof as Kurei. Kurei does have the clone technique though so Iori would be up against more than one opponent.