I would say that 3000 l/h is the min for a real good flow across the bottom, and by the time it gets to the end of a 1200 long tank there wont be the same flow across the bottom. I would use a spraybar of some type to spread the flow more evenly across the width of the tank and keep it as close to the bottom as you can. I extended the return pipe on mine so that the spraybar is only about 30-40mm off the bottom of the tank and is sitting on the rocks.
Tank mates, well at the moment I have about 1/2 a dozen blond guppies that are growing up and 3 inangia
unfortunately the heater in my hill-stream tank got turned off while the tank was out in the shed over winter and I lost at least 3 of the loaches in it &c:ry