....Which means that from the seventh turn onwards, there is no more introduction/placement of the base tokens anymore.
That's correct (seventh turn onwards if there's one Imperial player; fourth turn onward if there are two).
I suppose no movement of the base from planet to planet are allowed then?
Also correct -- in the sense that the Imperial player(s) cannot move a base.
That is once the base is placed at the planet, it stays there til the end of game unless it is destroyed.
Correct with one exception: if the Hutts destroy an Imperial Base, the base token is immediately moved to another Imperial planet. See page 18:
If a Hutt destroys an Imperial Base