Sage mode - is simply being able to knead nature energy and use it to boost your bodily functions
senjutsu - techiniques that involve nature energy ex. sage art rasengan
Ridoku sage mode is simply adding tailed beasts chakra to nature energy and regular chakra to create a new senjutsu or enchance an existing techinique
Sage mode is an actual mode, that one attains when he balances out the senjutsu energy.
However, we have noticed that those who become the Juubi's jinchuriki already have senjutsu at their possession and hence have no need to gather senjutsu energy from the outside. Meaning no need for Sage mode.
So with this its the same for Naruto. He isn't gathering the nature energy like normal, hence he doesn't have a sage mode, he is similar to the juubi's jinchurikis who also possess senjutsu chakra, but have no sage mode.