Steve;
Centrifugal pumps are sort of "self-regulating" but in a very defined manner, depending upon the specific pump design. In general, the more flow, the less the HP required. In general, the more head (i.e. pressure developed across the pump) the more HP required. But the pumps must operate within their flow and head design envelope or bad and unpredictable things happen (cavitation, fluid overheating, etc.)
But within the realm of centrifugal machinery, there are exceptions to the above and without seeing the pump curves, I cannot tell you how your pumps will perform. Being bilge pumps, they most likely have an open impeller and act more like vertical turbine pumps which often require MORE HP with more flow. This is what I would assume for your design purposes.
Your pump supplier may be able to supply you with a curve and get it if you can. But even without a curve, a bilge pump should have a specification that says something like: "350 gpm at 20 feet head". What that means is that when the system head (pressure drop from pump discharge to pump suction) is 20 feet (8.65 psi), then the pump will move 350 gpm. If the system head is less than 20 feet, the pump will move more fluid. If the system head is more than 20 feet, the pump will move less fluid.
In a situation like this, it is best to make your flow loop with as large a piping as practical and as few fittings as possible to minimize system head loss (pressure drop) and then install a valve at the outlet of the pump(s) so that if excessive flow occurs you can then throttle things back and get the pumps back operating on their curve.
As a general design guide, try to design your piping system to average 5 feet per second or less fluid velocity at the desire flow rate.
"Cameron's Hydraulic Handbook" is good reference for primary information on pumps and piping systems.
Ed in Florida