Do you know what the requirements of the venturi nozzle are? I ask because trash pumps do not make much pressure and the nozzle manufacturer calls the connection a "pressure hose" . I imagine that connection wants high flow backed by higher pressure. On top of that you are talking about reducing the discharge size further. This will reduce the water flow through your pump/system. If you have not contacted the nozzle supplier, I would ask them what kind of pump parameters are required for a functional venturi.

Fish on!,
Noel