This script is useful to spawn objects from a list onto Terrain.
I didn't work out how to get random x-rotation but maybe you will find it useful nonetheless.
Thanks to the Unity community for help along the way.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class SpawnTogether : MonoBehaviour
{
    public GameObject[] myObjects;
    public int cactusNumber;
    void Start()
    {
        CreateCacti(cactusNumber);
    }
    void CreateCacti(int cactiNum)
    {
        for (int i = 0; i < cactiNum; i++)
        {
            //generate int from the array
            int randomIndex = Random.Range(0, myObjects.Length);
            Vector3 randomSpawnPosition = new Vector3(Random.Range(-300, 300), i, Random.Range(-300, 300));
            //pass random spawn position to terrain height at chosen spot
            float spotHeight = Terrain.activeTerrain.SampleHeight(randomSpawnPosition);
            //create final position and feed in the values we generated
            Vector3 pos = transform.position;
                        pos.y = spotHeight;
                        pos.x = randomSpawnPosition.x;
                        pos.z = randomSpawnPosition.z;
            Instantiate(myObjects[randomIndex], pos, Quaternion.identity);
        }
    }
}