Ted, are you thinking the airlift pump will be almost horizontal, or is it vertical with an elbow near the water surface to transition to horizontal? I've never seen a nearly horizontal airlift pump, only vertical. Not saying it wouldn't work, but if efficiency is a main driver, you might want most of the "working part" of the pump vertical so the bubble flow interacts with the water column.
Larger airlifts might be less efficient, but you would need far less of them, so I would likely go that route. Just balancing out air flow to 20 separate pumps would be a trick. Maybe 6" pipe size would be good. I would likely run one air line to a point close to the pumps, then have a manifold with individual valves for each pump with some means of observing air flow to each pump to balance the flows out. Or you could use an orifice or other pressure equalizing approach, although that would reduce efficiency as well.
Depth to introduce the air for greatest efficiency involves the compressor as well. 3'feet of water head is very low pressure for most aeration type pumps. I don't think they would be very efficient in that range. Regenerative blowers are very efficient at moving lots of air at low pressure, so that might be an option. If you go that route, make sure you can control the pond depth, as a foot of additional water pressure could exceed the blower's max pressure, reducing flow to zip.
Good luck - hopefully we'll see the river in action!