I went ahead and added a 3" Oscar on December 31. As soon as he hit the water he swam right up next to the cholcoate, the largest fish in the tank at 8". 3 days later, he hasn't left the chocolates side. They swim together, they float together, they eat together. It's like they were long lost buddies. Kinda wierd actually. The Jack Dempsey could care less. Totally ignores them. Seems like all is well.
I filter at a flow rate of 700 GPH, with 2 Magnum 350 filters and bio wheels across the back. Typically do a 50% water change every 3-4 weeks. Been running great for about 4 years. When the Oscar get's bigger may have to increase the water changes.