You cannot make a pages doc on the ipad in landscape mode initially. you have to have a premade document from a desktop version made in landscape mode and opened on the ipad later. Save this blank copy for use as a source for new documents.
You cannot mix portrait and landscape in pages in iOS, dont know about desktop.
You have to use other app like Polaris Office. There you have the option to change the page layout. Open the document in Polaris office as word document. Then change the page layout from Portrait to Landscape or vice versa. Then again open the same document in Pages. Pages has very nice userfriendly features. It lacks only this feature.