How to find the active random object in an object pool array

Hi there guys.

I have a bit of a problem with a script I use to generate an endless levels. The original script uses instantiation to add the new level as the player moves in the Y direction (up). But the object being instantiated does too much heavy lifting, and I get fragmentation when a new part of the level or a new "room" is instantiated, which is why I would like to pool the prefab objects and just set them active as needed.

This is my original level generation script:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class RoomGenScript : MonoBehaviour {
 
     public GameObject [] availableRooms;
     public List<GameObject> currentRooms;
     private float screenHeightInPoints;
 
     // Use this for initialization
     void Start () {
         float width = 1f * Camera.main.orthographicSize;
         screenHeightInPoints = width*Camera.main.orthographicSize / 64f;
     }
    
     // Update is called once per frame
     void Update () {
 
     }
 
     void FixedUpdate()
     {
         GenerateRoomIfRequired ();
     }
 
     void AddRoom(float farthestRoomEndY)
     {
         int randomRoomIndex = Random.Range (0, availableRooms.Length);
         GameObject room = (GameObject)Instantiate (availableRooms [randomRoomIndex]);
         float roomHeight = room.transform.FindChild ("floor").localScale.y;
         float roomCenter = farthestRoomEndY + roomHeight * -0.678f;
         room.transform.position = new Vector3 ( 0, roomCenter, 0);
         currentRooms.Add (room);
     }
 
     void GenerateRoomIfRequired()
     {
         List<GameObject> roomsToRemove = new List<GameObject> ();
         bool addRooms = true;
         float playerY = transform.position.y;
         float removeRoomY = playerY - screenHeightInPoints;
         float addRoomY = playerY + screenHeightInPoints;
         float farthestRoomEndY = 0;
         foreach (var room in currentRooms) {
             float roomHeight = room.transform.FindChild ("floor").localScale.y;
             float roomStartY = room.transform.position.y - (roomHeight * 0.1f);
             float roomEndY = roomStartY + roomHeight;
 
             if (roomStartY > addRoomY)
                 addRooms = false;
             if (roomEndY < removeRoomY * 1f)
                 roomsToRemove.Add (room);
             farthestRoomEndY = Mathf.Max (farthestRoomEndY, roomEndY);
         }
 
         foreach (var room in roomsToRemove) {
             currentRooms.Remove (room);
             Destroy (room);
         }
         if (addRooms)
             AddRoom (farthestRoomEndY);
     }
 }

So I've tried my hand at pooling the objects and then just setting them active, but I'm missing something and just not sure exactly how to go about it. In my modified script, I've instantiated the objects in the start menu and then gotten rid of the instantiation in my "AddRoom" function, and set a random object from the array to active. Now Im not sure how to add the random active "room" to my "currentRooms" List in the "AddRooms" function.

This is the modified script:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class RoomGenScript : MonoBehaviour {
 
     public GameObject [] availableRooms = new GameObject[4];
     public List<GameObject> currentRooms;
     private float screenHeightInPoints;
     public GameObject room1;
     public GameObject room2;
     public GameObject room3;
     public GameObject room4;
     // Use this for initialization
     public void Start () {
         float width = 1f * Camera.main.orthographicSize;
         screenHeightInPoints = width*Camera.main.orthographicSize / 64f;
 
             
         availableRooms [0] = (GameObject)Instantiate (room1);
         availableRooms [1] = (GameObject)Instantiate (room2);
         availableRooms [2] = (GameObject)Instantiate (room3);
         availableRooms [3] = (GameObject)Instantiate (room4);
         room1.SetActive (false);
         room2.SetActive (false);
         room3.SetActive (false);
         room4.SetActive (false);
         }
 
     
     // Update is called once per frame
     void Update () {
 
     }
 
     void FixedUpdate()
     {
         GenerateRoomIfRequired ();
     }
 
 
 
     void AddRoom(float farthestRoomEndY)
     {
         int randomRoomIndex = Random.Range (0, availableRooms.Length);
         availableRooms [availableRooms.Length].SetActive (true);
         float roomHeight = transform.FindChild ("floor").localScale.y;
         float roomCenter = farthestRoomEndY + roomHeight * -0.678f;
         transform.position = new Vector3 ( 0, roomCenter, 0);
         currentRooms.Add (//What do I add in here?);
     }
 
     void GenerateRoomIfRequired()
     {
         List<GameObject> roomsToRemove = new List<GameObject> ();
         bool addRooms = true;
         float playerY = transform.position.y;
         float removeRoomY = playerY - screenHeightInPoints;
         float addRoomY = playerY + screenHeightInPoints;
         float farthestRoomEndY = 0;
         foreach (var room in currentRooms) {
             float roomHeight = room.transform.FindChild ("floor").localScale.y;
             float roomStartY = room.transform.position.y - (roomHeight * 0.1f);
             float roomEndY = roomStartY + roomHeight;
 
             if (roomStartY > addRoomY)
                 addRooms = false;
             if (roomEndY < removeRoomY * 1f)
                 roomsToRemove.Add (room);
             farthestRoomEndY = Mathf.Max (farthestRoomEndY, roomEndY);
         }
 
         foreach (var room in roomsToRemove) {
             currentRooms.Remove (room);
             Destroy (room);
         }
         if (addRooms)
             AddRoom (farthestRoomEndY);
     }
 }

I feel like I'm looking at this the wrong way. Any guidance would be greatly appreciated. Thank you in advance.