If you are going to insert documents that vary in orientation, you are going to have to identify to the macro which documents are portrait and which are landscape. One way to do that is to insert 'landscape' into the filename. You can then modify the code to detect that name and set the orientation for the added section accordingly e.g.
Private Sub CommandButton1_Click()
sPath = ThisDocument.Path & "\Templates\"
With ActiveDocument
For i = 0 To ListBox1.ListCount - 1
If ListBox1.Selected(i) = True Then
Set oRng = .Range
With oRng
.Collapse wdCollapseEnd
.InsertBreak wdSectionBreakNextPage
.End = ActiveDocument.Range.End
.Collapse wdCollapseEnd
With .PageSetup
If InStr(1, LCase(ListBox1.List(i)), "landscape") > 0 Then
.Orientation = wdOrientLandscape
Else
.Orientation = wdOrientPortrait
End If
.DifferentFirstPageHeaderFooter = False
End With
.InsertFile sPath & ListBox1.List(i) & ".docx"
End With
End If
Next i
End With
Unload Me
End Sub
I don't understand your comment about content controls, but if you add documents that have content controls, any macro that references content controls will need to know about those additional controls.