Do characters with the Gift who have their arts opened to Elementalists have a 0 in all forms and all techniques? Or just a 0 in all forms?
There is no special treatment between the tyes of Arts during opening - the 15 are opened at once with a score of zero (the apprentice does not get x in that season of teaching). see p.106, "Thus, in this season the apprentice gains a score of 0 in all fifteen Hermetic Arts, but learns nothing else."
If you're speaking of the Hedge magic revised elementalists, they'd get all of their applicable arts for their individual tradition opened. They'd get 0 in everything, (note that none of the example traditions have a complete set of elementalist abilities).