My Dovii have never shared a tank, so I probably wouldn't recommend it. As for my Umbee pair. They shared their tank with a breeding pair of 7" Jags. The Jags spawned several times, and stayed to their side of the tank, and the Umbee stayed on theirs. Once my Umbee spawned, it was a different story, he started taking on both Jags. I Quickly removed the Jags. Really couldnt tell which fish was in more danger. The Jags fought as a team, while the Umbee male was on his own.
Long story short. Keeping them with others while young should be OK. Once they pair up or grow up, better be careful.
Got any pics? Sounds like some great fish. Yea I would say all bets are off when they mature.......12" or 18" Umbee and Dovii won't play nice. If you have a huge tank they may not kill everything.......I have seen in a public aquarium a monster 24" dovii own half a tank. I would guess it was 20' or more and 6' deep.