Using Google AdMob

1st question: No, at least I do not recommend it is good to make separate ad units for each activity, in applications you work with fragments you can place the ad for example along with the container that carries the fragment, but in the case of the interstitial it is better to make separate blocks.

2nd question: Usually the ad takes a while to be uploaded, this can happen to several factors either because it is a video advertisement or the internet of the user, the solution that I will give you may not be the best but solves your problem, I made the part of the showInterstitial () it opens the interstitial and returns true, returning it loads the code to download the firebase's database.

    public class MainActivity extends AppCompatActivity {

private InterstitialAd mInterstitialAd;
private boolean boleanloaded;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    boleanloaded = false;

    MobileAds.initialize(this, "ca-app-pub-5718158120252618~4586568420");

    mInterstitialAd = new InterstitialAd(this);
    mInterstitialAd.setAdUnitId("ca-app-pub-3940256099942544/1033173712");
    carregarAnuncio();

    mInterstitialAd.setAdListener(new AdListener(){
        @Override
        public void onAdClosed() {
            carregarAnuncio();
        }
    });

    while (!boleanloaded) {
        boleanloaded = showInterstitial();
        if (boleanloaded)
        banco.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                mInterstitialAd.show();
                onibus.clear();
                for (DataSnapshot data : dataSnapshot.getChildren()) {
                    BusAdmin b = data.getValue(BusAdmin.class);
                    b.setKey(data.getKey());

                    onibus.add(b);
                }
                adapter.notifyDataSetChanged();
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }

        });}
    }


}

private void carregarAnuncio() {
    mInterstitialAd.loadAd(new AdRequest.Builder().build());
}

private boolean showInterstitial() {

    if (mInterstitialAd != null && mInterstitialAd.isLoaded()) {
        mInterstitialAd.show();
        return true;
    } else {
        return false;
    }
}

I do not understand AdMob , but to know that the connection to Firebase Database is established, there is a very easy way. It's well explained in the Google development guide under How to detect the connection status from this link . Using the code

DatabaseReference connectedRef = FirebaseDatabase.getInstance().getReference(".info/connected");
connectedRef.addValueEventListener(new ValueEventListener() {
  @Override
  public void onDataChange(DataSnapshot snapshot) {
    boolean connected = snapshot.getValue(Boolean.class);
    if (connected) {
      System.out.println("connected");
    } else {
      System.out.println("not connected");
    }
  }

  @Override
  public void onCancelled(DatabaseError error) {
    System.err.println("Listener was cancelled");
  }
});

Putting a while(!connected){...} I think it solves your problem. Ps: Remember to use the database reference in the root of your database as in the example, entering a child will not work.