I'd say whether you empty it or not depends on which hose you're taking off. I am using the stock temp probe for the stock electric fan. Ran a wire from the positive lead of the stock fan to the aftermarket fans relay as my switch. When the stock fan comes on at 220°F or when the AC compressor is on the both fans kick on. Otherwise it's quiet as anything.
Yes you can get rid of the clutch pulley and even reroute the original belt and it will fit without measuring and getting new belts. The reason most don't take it off is because it's a pain in the butt and it doesn't really have any drag. All the drag was in the fan.
91 Cherokee with selectrac, 2" Rusty's AAL BB, 31x10.5 Liberator AT's