The most important factor to consider on your cockatiel's nutrition and diet is variety. Your cockatiel will dictate what it likes and what it dislikes. It's important that you never force your cockatiel to eat something it does not want, again your cockatiel will select on its own what it likes. If your cockatiel only eats just a small amount of any other food besides its regular seeds or pellets diet, that is OK too, again the importance here is variety on your cockatiel's diet.
When serving your cockatiel fruits and veggies, it will be best to serve small amounts, either chopped, shredded or in very small pieces, this will motivate them to grab them and give them a try.