Unet Spawn Prefabs When Scenes Changes

I am extending the Network Manager script in an attempt to spawn enemy prefabs at specified locations on my scene. In my scene I have a game object name "meta_enemies" that contains child GameObjects with transforms that serve as spawn points for enemies. I would like to spawn enemies on the server when the scene loads, but I cannot call NetworkServer.spawn from my network manager script.

Here is my Network Manager:

 using UnityEngine;
 using UnityEngine.Networking;
 using System.Collections;
 
 public class CustomNetworkManager : NetworkManager {
 
     // Overide Functions _________________________________________________________
     public override void OnStartServer()
     {
         base.OnStartServer();
         autoCreatePlayer = true;
     }
 
     public override void OnServerSceneChanged(string sceneName)
     {
         base.OnServerSceneChanged(sceneName);
 
         GameObject enemies = GameObject.Find("meta_enemies");
 
         foreach (Transform child in enemies.transform) {
             if (child.name == "imp")
             {
                 //Spawn imp enemies @ transform position
             }
 
             print("Foreach loop: " + child);
         }
             
     }
 //___________________________________________________________________________
 }
 

Here are my questions:

1. How should I spawn enemies when the scenes change?

2. How do specify with prefab I spawn from the list of spawnable prefabs?