[mark]

Sure one pump.

Run the pump into a manifold with a valve on each leg (tank, skimmer, chiller, spare) to adjust flow and to isolate. Might just take a bit to adjust to the desired volume in each leg as the interaction between each. Finer control with gate valves over ball valves.

Would also be able to remove skimmer and chiller as well without having to shut off return, just would probably need to adjust the valves on the active legs.

Problems I see is; would you have enough volume in the pump compartment to handle to flow and enough room in the sump for all the water if you lost power or took your skimmer and chiller off line.

As you suggest should be no problem with the 1" return since you only planning on 600gph back from you tank. Wouldn't reduce to 3/4" line as would limit if plans change and would be using the valve to reduce flow to tank anyways. Can restrict the outlet of pumps not the inlet.