I found an answer. The answer is that DIAPHRAGM PUMPS are the most gentle. This animated GIF makes it pretty clear why they are so gentle. http://en.wikipedia.org/wiki/File:DiaphragmPump.gif
I cannot imagine that a diaphragm pump would ever harm non-strand-forming, small diameter algae. All the pump does is apply a negative pressure and a positive pressure. The algae just go through two gates. And that's it. A centrifugal pump, however, should probably be avoided, though perhaps such algae would not even be harmed by such pumps because they are so small. You would just have to try and see.