How to spawn random enemies at a free random spawn point?

Hi I'm trying to have random enemies spawn at random free spawn points. I've got it to work to the point where I can spawn random enemies at random spawn points but having hard time scripting it to only spawn at a free spawn points.

This is the script for checking for a free spawn points.This is the first time I'm trying to use Arraylist and I'm totally lost :(

    var spawnPoints : Transform[];  
    var enemyPrefabs : GameObject[]; 
    var amountEnemies = 20;  
    var yieldTimeMin = 1; 
    var yieldTimeMax = 3; 

    private function AvailableSpawnPoints() : GameObject[] //checks for available spawnpoints
        var gos : ArrayList = new ArrayList();

        for( cnt = 0; cnt < spawnPoints.length; cnt++ )
            if (spawnPoints[cnt].transform.childCount == 0 )

        return gos.ToArray();

And this is to actually spawning enemies,

function Spawn() 

   var gos : GameObject[] = AvailableSpawnPoints();

   for (i=0; i< gos.length; i++) 
      yield WaitForSeconds(Random.Range(yieldTimeMin, yieldTimeMax));  

      var obj : GameObject = enemyPrefabs[Random.Range(0, enemyPrefabs.length)]; 

      Instantiate(obj, gos*.transform.position, Quaternion.Identity);* 
 _obj.transform.parent = gos*.transform;*_
_*<p>Any ideas? Thanks.</p>*_

Have a similar problem. Here's what I've used till now.

I have 2 arrays, one comprised of the possible collectibles to be spawned randomly and another comprised of the empty tiles (where a spawning can occur).

The problem is that the game is an iOS game and the way I update the contents of the empty tiles array is cpu-intensive and the game is pretty much unplayable at the moment.

If you're targeted to pc/mac/web though, this method works fine.