A dovii is no "large powerful open water predator,capable of very fast bursts of speed", compared to any big hap, it is a slow sedentary slug. I wouldn't put it in with tankmates in anything less than at least 10 feet, but I think you are glorifying dovii beyond what it deserves. A dovii is a big, mean, ugly, slow CA cichlid, in fact the only reason people keep them is because they are the biggest, ugliest and meanest CA cichlid. On its own scale, and considering its fighting ability, a betta may be jusr as aggressive. The only difference is people know how to cohabit them and don't know how to do it with dovii. If you weren't afraid of losing a few fish, you probably could figure it out.