Terrain Brush for buildings instead for trees bushes etc.

No you cannot have prefabs as your mesh for the terrain editor. What I did to get around a similar problem was code my own editor script that automatically mass placed a prefab for me. These are the related files:

 using UnityEngine;
 
 public class StarPlacementHelper : MonoBehaviour
 {
     public Color starTint;
     public Color starTint2;
     public float minDensity;
     public float maxDensity;
     public Bounds bounds;
     public SpriteRenderer[] stars;
     public Vector2 minSize;
     public Vector2 maxSize;
     public bool randomRotation;
 
     public void Place()
     {
         if (minDensity > maxDensity || maxDensity < 0)
             return;
 
         float xHalfExtent = ((float)bounds.extents.x / 2);
         float yHalfExtent = ((float)bounds.extents.y / 2);
         float zHalfExtent = ((float)bounds.extents.z / 2);
         for (float x = bounds.center.x - xHalfExtent; x < bounds.center.x + xHalfExtent; x += getRandomDensity())
         {
             for (float y = bounds.center.y - yHalfExtent; y < bounds.center.y + yHalfExtent; y += getRandomDensity())
             {
                 //skip factor
                 if (Random.Range(0, 9) > 1) //80 percent
                     continue;
 
                 for (float z = bounds.center.z - zHalfExtent; z < bounds.center.z + zHalfExtent; z += getRandomDensity())
                 {
                     //skip factor
                     if (Random.Range(0, 9) > 1) //80 percent
                         continue;
 
                     int index = Random.Range(0, stars.Length);
                     Color col = Color.Lerp(starTint, starTint2, Random.Range(0f, 1f));
                     SpriteRenderer r = Instantiate(stars[index], new Vector3(x, y, z), Quaternion.identity) as SpriteRenderer;
                     r.color = col;
                     r.transform.localScale = getRandomSize();
                     r.transform.parent = transform;
                     if (randomRotation)
                         r.transform.eulerAngles = new Vector3(0, 0, Random.Range(0f, 359f));
                 }
 
             }
         }
     }
 
     public void DeleteChildren()
     {
         //foreach (Transform t in transform)
         //    DestroyImmediate(t.gameObject);
 
         foreach (SpriteRenderer t in transform.GetComponentsInChildren<SpriteRenderer>())
             DestroyImmediate(t.gameObject);
     }
 
     private Vector3 getRandomSize()
     {
         //we want a square anyways
         float x = Random.Range(minSize.x, maxSize.x) + minSize.x;
         //float y = Random.Range(minSize.y, maxSize.y) + minSize.y;
         //return new Vector3(x, y, 0);
         return new Vector3(x, x, 0);
     }
 
     private float getRandomDensity()
     {
         return Random.Range(minDensity, maxDensity);
     }
 
     void OnDrawGizmosSelected()
     {
         Gizmos.color = new Color(1, 1, 1, 0.3f);
         Gizmos.DrawCube(bounds.center - Vector3.forward * 20, bounds.extents);
     }
 }



 ///////////////
     //place this one inside of an editor folder:
 ///////////////
     using UnityEngine;
     using UnityEditor;
 
 [CustomEditor(typeof(StarPlacementHelper))]
 public class StarPlacement : Editor
 {
     public override void OnInspectorGUI()
     {
         serializedObject.Update();
 
         StarPlacementHelper s = target as StarPlacementHelper;
 
         base.OnInspectorGUI();
 
         if (GUILayout.Button("Populate"))
         {
             s.Place();
         }
         else if (GUILayout.Button("Delete Children"))
         {
             s.DeleteChildren();
         }
    }
 
     private static void Space(int num = 1)
     {
         for (int i = 0; i < num; i++)
             EditorGUILayout.Space();
     }
 }

Keep in mind that this was for a 2D project, you may have to change up some of the axis to get what you are looking for.