Both venturi and needle wheel skimmers use a venturi to draw in air. It is based on a fluid mechanics principle called Bernoulli's theorem which basically says that an increase in fluid velocity is matched by a corresponding reduction in pressure. The venturi accelerates the water in the throat which reduces the pressure and draws the air in. The main difference is that the classic venturi has the venturi on the discharge side of the pump and the needle wheel has the venturi on the suction side of the pump. The needle wheel also has the "needle" impeller to break up the air bubbles into smaller sizes. I believe that because the venturi on a needle wheel operates at lower pressure on the suction side, it can draw in more air with less power than if the venturi is on the discharge. The disadvantage is that all that air in a pump is not good for it mechanically and results in shorter life, and because of all the air you cannot have a tall skimmer as the pump will struggle to develop much head with all that air in the suction. The classic venturi is more trouble free but will use more power for similar performance.
Doing a DIY skimmer is certainly possible, but it is a lot of trial and error to find a pump and column with the right mix of water, air, skimmer diameter and height. Not sure it is worth the trouble unless are the type of person that just likes to tinker and experiment with stuff like this. You will likely spend more money on the DIY in the end as you are likely to have to purchase several pumps.
