Just to qualify my answer: I work at SG-1 in the art dept.
Could a helicopter wit retracted rotors fit through the gate?
Absolutely. I worked on the .dxf files to manufacture the stargate, its plenty big enough at it's widest point to allow passage of very large cargo, sorry I can't remember the inside diameter at the moment,but it is sizeable.
As for getting a chopper or other large items into the gateroom, one detail you may have forgotten is that the gate is housed 28 floors underground in a deactivated missle silo, so although it would not be easy, anything that could be lowered by crane that could fit through an open icbm missle bay and did not exceed about forty feet in length (the approximate distance from the exterior control room wall to an event horizon surge) could be transported through the gate.
Glad to be of service.