It is not necessary to keep all of the impellers at the same diameter. However, it is better to limit the extent of impeller reduction to five percent or less due to loss in efficiency as the impeller is cut down. You should evaluate the possible loss in efficiency by cutting only one or two stages if the reduction is more than five percent. Regarding cutting of impeller shrouds, the advantage is that the full shroud diameter will minimize pump instability when it is operated below fifty percent of its best efficiency point. However, cutting the shrouds will reduce the impeller disc friction and improve efficiency.
Centrifugal pumps can usually handle up to about five percent by volume of air.
Above that, they will easily become airbound, especially at flow rates below the best efficiency point. Even when pumping five-percent air, the pressure developed will be reduced due to the reduced specific gravity of the fluid mixture. Add a bypass pipe to maintain a high rate of flow. Install a vent in the top of the suction pipe to vent air back to the source. In addition, the condition known as suction recirculation becomes very pronounced at very low flows causing vortices in the impeller and localized pitting damage to the impeller blades.