One of my favourite things about Osphronemus is the fact that they (usually) allow for a tank containing a nice big fish combined with much smaller ones without worrying about predation. In most cases that only works when the small fish are
so small that they are beneath the notice of the big predator; I like the mix in your tank with those blue gouramis and loaches. If the Osphro were actually a predator, he'd consider those other fish big enough to be worthwhile eating.
I like your notion of fresh-water "live rock" as well. The typical reaction that we read about all too often, when some tiny critter finds its way into a tank, is something like "OH MY GOD! WHAT IS THIS THING?? How do I kill it? I can't bare to even think that I am sharing the planet with such disgusting creepy-crawlies!
HELP!!!" I always wonder why people like that ever got started in the aquarium hobby in the first place.
But, although I like the idea of bio-diversity in a tank, and I agree that those little feral hitchhikers probably assist in the breakdown of some waste and missed food crumbs, I doubt that they contribute anything to reduced nitrates; they may very well assist in and speed up the initial breakdown, making the nutrients more easily available to bacteria. If anything, that would speed up nitrate accumulation.
You'd need a large mass of algae or plants to utilize that nitrate to any noticeable extent, and that would be pretty difficult with a hungry Osphro in the tank, unless you have room in a large sump for the plants and/or some kind of anoxic denitrifying filter medium. Personally...I'd just keep changing water.
in the coming year
