A 36" long tank doesn't allow much swim space for a 12" Dovii. It can cover that in a second at that size. I would say have the means and be ready to upgrade when it is needed. Ball park 8-10" for a Dovii in that footprint.....nice width though. For the life of a Dovii, IMO 8' x 36" is a footprint minimum. My male is getting close to 3 years old and 17" or more, in a 180....I need to set up his 360 soon. There is big difference between a 4 year old 19" Dovii and a 10 year old Dovii given the proper housing. IMO a singe male would be good for a few years in a 6' x 24' with weekly water changes, good filtration and a healthy diet. It will vary as always with the fish and care though.
I got my male when he was a little past free swimming. 1/4" or so and 17"+ in 3 years. He hit 12" around a year and half and slowed a little bulked up. A 12" dovii is a lot more fish then a lot of 12" fish.