ASTM5092 standard for well design and construction sets the maximum entry velocity of the water in the well at 3 centimeters per second.  How does that limitation affect the velocity at which water enters the sampling pump?

The biggest concern here is what do we mean by low flow? It is not just the flow rate measured at the wellhead but it’s the flow of water through the well screen. What we are trying to achieve is to not greatly exceed the ambient groundwater flow. That established limit of about 3 centimeters per second through the screen zone is to try to maintain laminar flow through the screen and into the well bore. 1/10 to 3/10 of a foot per second is about the limit for maintaining that laminar flow.