do i need a cooling system resovior ?

As long as the overflow tube is below the water level and the cap is correct for a return system, yes.

Those "engineered" bottles that have the hose connection plumbed to the bottom of the container guarantee the hose is always at the lowest level.

I have mine cut so that it is about 3/4 of an inch from the bottom of the bottle.

Works very well.

I try to keep the bottle 1/2 full so that it will hold overflow and also have coolant to suck back in.

Adding the bottle has lengthened the time from checking the radiator level from every drive to about every 5th or 6th drive, and instead of removing the cap, simply glancing at the bottle.