It is possible that the error messages you were getting were because you were calling the "isHit" method incorrectly. In your current code, you'd have to call it like this:

if (someEnemyObject.isHit(someBulletObject, someEnemyObject, someBoolean))
{
   // There was a hit!
}

Note that the boolean parameter doesn't do anything (it is assigned to and returned), so we can remove it from the "isHit" method:

public bool isHit(Bullet b, Enemy e)
{
    if (b.position.X == e.ePosition.X && b.position.Y == e.ePosition.Y)
    {
        return true;
    }
    else
    {
        return false;
    }
    
}

Now to call it:

if (someEnemyObject.isHit(someBulletObject, someEnemyObject))
{
   // Hit!
}

It is now a bit odd to have to pass the enemy object to the method in two ways, the explicit parameter but also using the "this" mechanism (by calling the method on a given object).

There are a couple of solutions to this, but given that this logic crosses two different objects, we can just move this logic into the game class where the two different objects are interacting anyway:

private bool isHit(Bullet b, Enemy e)
{
    if (b.position.X == e.ePosition.X && b.position.Y == e.ePosition.Y)
    {
        return true;
    }
    else
    {
        return false;
    }
    
}
 
 
protected override void Update(GameTime gameTime)
{
    if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed || Keyboard.GetState().IsKeyDown(Keys.Escape))
        Exit();
 
    if (Keyboard.GetState().IsKeyDown(Keys.Space) && pastKey.IsKeyUp(Keys.Space))
    {
        Shoot();
    }
 
    pastKey = Keyboard.GetState();
 
    // Level Check ----- Level Check ----- Level Check ----- Level Check ----- Level Check -----
 
    Enemyss();
 
    // Updates
    UpdateBullet();
    pla.Update();
    foreach (Bullet bullet in bullets)
    {
        foreach (Enemy enemy in enemies)
        {
            if (isHit(bullet, enemy))
            {
                // There was a hit!
                // Maybe do something like enemy.TakeDamage();
            }
        }
    }
    base.Update(gameTime);
}

If your game class starts getting very long, you can move all the bullet and enemy game logic handling into a separate class, e.g. "World" or something.

A god design rule of thumb is to try not to let too many classes know about the overall program state. So try keep the bullet and enemy class to simple objects that don't really know or care about the existence of other game objects, and just rely on higher level parts of the program to inform them about interesting events from the outside world (like having an enemy.TakeDamage() method for example).

Yes, you definitely don't want to use floating point equality. I was going to mention about bounding box collisions but it slipped my mind, but Googling for "XNA sprite collision" or "XNA bounding box collision" should return some useful results.

In addition to the accuracy issue when comparing position, there's also this:

Enemy enemy = new Enemy(Content.Load("Enemy"), new Vector2(e_x, 100), new int());

//Other code goes here...

foreach (Bullet bullet in bullets)
{
	if (enemy.isHit(bullet, enemy))
	{
		enemy.Health -= 5;
		bullet.isVisible = false;
	}
}

You aren't testing against the enemies in your "enemys" list.

You're testing to see if the bullets collide with a newly created enemy.

You need to check every bullet against every enemy in your "enemys" list*.

It also seems wrong to me creating a new instance of Enemy every time the function is called, and then only adding it if less than 3 exist in your list.

Check first, then only create & add if needed.

*Note that there are better ways of dealing with this later on, but for now a brute force way should be sufficent.