the idea of throwing a rasengan wouldnt work imo because you need to keep the form of the chakra as a sphere. how are you gonna control that once you throw it and its out of your control.

The thing about the jounin and that they ATLEAST have 2 elements (note the fact i caped it) Would mean they can have more then 2 maybe even all of them. One will have to dominiant of course but other then that i wouldnt be surprised to see many jounin lvl ninja using multiple elements easy.

Now to be able to change the form and use the element of the a jutsu is completely diffrent ie: chidori because this is a whole new lvl of using elements. just put it this way even though naruto isnt a fire user he can probably learn how to use the fire spitting techniques sasuke uses, but i doubt he will be able to add the fire elements abilty to another jutsu like rasengan.