SQL inventory Questions One server, multiple instances

Good afternoon scripting experts!  

I am a new DBA for SQL and one of my first projects is getting a hand on the inventory of all the servers/instances that we administer.  I have a script that I run that is a hodgepodge of a few different scripts I have found here and it works great with one exception.  (I am about to show my inexperience with the next part, please forgive me)

If I go into management studio and type in connect to "GLDBA03102" it connects fine I have been calling that the "Server" If I go to a server that has "Servername\Instancename" (I hope thats correct) my program is not able to connect and it just gives me the generic error message that it is unable to connect - Is there a Way I can roll through all "Instances" on a specfiic server?  Currently I'm using a serverlist.txt file that just shows all server names....   The problem is some of these servers are "virtual" and contain many instances, and I dont trust another DBA will provide me with all the instances, which is why I would prefer to scan them instead of "hardcoding" them into my serverlist.txt....

I hope this makes sense, I'm sorry for the incorrect verbage.. Include are a few samples to hopefully help!

My main Script (I am sorry for not having the original authors name, I have hacked this thing a bit and did not retain it!)

#region Variables
$TextFileLocation = "C:\serverlist.txt"
$intRow = 1
$BackgroundColor = 36
$FontColor = 25
#endregion

#Region Open Excel
$Excel = New-Object -ComObject Excel.Application
$Excel.visible = $True
$Excel = $Excel.Workbooks.Add()
$Sheet = $Excel.Worksheets.Item(1)
#endregion


#Go through text file one at a time
foreach($instance in Get-Content $TextFileLocation)
{
     $Sheet.Cells.Item($intRow,1) = "INSTANCE NAME:"
     $Sheet.Cells.Item($intRow,2) = $instance
     $Sheet.Cells.Item($intRow,1).Font.Bold = $True
     $Sheet.Cells.Item($intRow,2).Font.Bold = $True
	 for ($column =1; $column -le 2; $column++)
	 {
	 $Sheet.Cells.Item($intRow, $column).Interior.Colorindex = 44
	 $Sheet.Cells.Item($intRow, $column).Font.ColorIndex = $FontColor
	 }
 	 #Increase Row count by 1
     $intRow++
	 #Create sub-headers
      $Sheet.Cells.Item($intRow,1) = "Name"
      $Sheet.Cells.Item($intRow,2) = "LAST FULL BACKUP"
      $Sheet.Cells.Item($intRow,3) = "LAST LOG BACKUP"

	#Format the column headers
	for ($col = 1; $col -le 3; $col++)
    {
          $Sheet.Cells.Item($intRow,$col).Font.Bold = $True
          $Sheet.Cells.Item($intRow,$col).Interior.ColorIndex = $BackgroundColor
          $Sheet.Cells.Item($intRow,$col).Font.ColorIndex = $FontColor
    }
	 #Finished with Headers, now move to the data
     $intRow++
	################################################
	[System.Reflection.Assembly]::LoadWithPartialName('Microsoft.SqlServer.SMO') | Out-Null

	 # Create an SMO connection to the instance in servers.txt
     $s = New-Object ('Microsoft.SqlServer.Management.Smo.Server') $instance

	$dbs = $s.Databases
	
	foreach ($db in $dbs)
	{
	 if (($db.Name -ne "tempdb") -and ($db.Name -ne "model") -and ($db.Name -ne "msdb") -and ($db.Name -ne "master"))
	 {
	 	 if($db.LastBackupDate -eq "1/1/0001 12:00 AM")
		{
		 	$fullbackupdate = "Never Backed Up"
			$fgColor = "red"
		}
		else
		{
		#$fullBackupDate= "{0:g2}" -f $db.LastBackupDate 
		$fullBackupDate = $db.LastBackupDate
		}
		$Sheet.Cells.Item($intRow, 1) = $db.Name 
    	$Sheet.Cells.Item($intRow, 2) = $db.LastBackupDate #$fullBackupDate 
		if ($db.RecoveryModel.Tostring() -eq "SIMPLE")
        { 
           $logBackupDate="Simple"
		}
		else
		{ 
	       #See Date Above..-eq is same as =
           if($db.LastLogBackupDate -eq "1/1/0001 12:00 AM")  
           { 
               $logBackupDate="Never" 
           } 
           else 
           { 
               #$logBackupDate= "{0:g2}" -f $db.LastLogBackupDate 
			   $logBackupDate = $db.LastLogBackupDate			   
           } 
    	  } 
    	$Sheet.Cells.Item($intRow, 3) = $logBackupdate
	    $intRow ++ 
       } 
   } 
   $intRow ++ 


}

$Sheet.UsedRange.EntireColumn.AutoFit()
cls

in my example on the excel spreadsheet it will not list any details for "Poly1", even though there are multiple instances on that server name