So...are you saying that they don't reach 20"+ because they're kept in 180's? If so I agree with you.
I've personally seen a pair of dovii (20"+ male, 8-10" female) kept in a 600 gallon tank. Alone. And it's one of the most impressive sights I've seen in fish keeping. The same guy is growing out a pair of umbee to keep in his other 600 gallon, they're not quite large enough yet.
So could you keep an umbee in a 180? Sure. It's your fish, do what you want. Is it ideal and will it allow the fish to grow to it's maximum potential? Probably not.