I don't know much about Golden Sun, but having Isaac use non-Venus elemental spells seems like a bad move. And it's also not comparable to Ness, Robin or Zelda.
Ness and Lucas borrow moves from their comrades, true, but they're just different psychic abilities that aren't really tied to the characters that use them (except PSI Rockin', which Ness still does not use
). Robin
can use Nosferatu, just not as a Tactician (and it still fits into the theme of him using different tomes).
And Zelda uses Din's Fire, Nayru's Love, and Farore's Wind, which are really just spells (supposedly) taught to anyone who can use them. They're not really Link exclusive. Not to mention Zelda IS a magic user besides.
Meanwhile, Isaac is (?) trained to use a specific type of magic, Venus, and everyone else specializes in other magic types so, having him be a godlike magician that can use every type of magic is not really accurate.
Also, imo, Isaac using every element in his moveset would bog him down and make him feel very generic. I think focus on stretching Earth magic to it's limits is a much better approach.