"If I keep my unit of Necron Immortals in Reserve, are they allowed to enter play via the Monolith's Portal?"
No, the Monolith clearly states "If a unit of Necron Warriors is eligible to enter play from Reserve" - this does not permit other units with the 'Necron' Special Rule to do the same. Note also, that unit of Necron Warriors must be declared to be entering play through the Monolith at deployment to be eligible for this method of battlefield insertion.
A bit on this one, Necron Warriors arriving from Reserves MUST be ported through the Monolith.
Strange, but not a Stranger.