A 180 would be ok for a single male dovii long term IMO. However I certainly do not think the clown knife will be tolerated. I always like to keep cichlids in pairs, think it's more natural. I would get a group of youngsters, maybe 6-8, more if you want, grow em' out in the 180, let a pair form and sell off the rest. I would keep the pair in the 180. Yes they will need a bigger tank eventually, but temp. the 180 will work. Then I would just plan on getting a bigger tank. The fish you like need BIG tanks. That is what I would do.