The rotational speed and impeller diameter are established when the pump is selected for a specific duty as determined by the purchaser and supplier. The effect of higher viscosity is to reduce the efficiency of the pump and, in turn, reduce the head an capacity output.
The maximum viscosity is determined by how badly the increase in velocity affects the efficiency and has no fixed number.
I dont understand this question, Im afraid.
As you can see, simply answering your questions will not give you the information that you are really seeking. I would suggest a course of study on pumps. There are some articles available on my website that may help. www.practicalpumping.com