Vegetables are a great healthy food, and including them in the diet is a huge improvement over feeding only seeds. But it's basically impossible to come anywhere close to the ideal balance this way unless you're a nutritionist or have a mash recipe where somebody has done the work of figuring out what it takes to achieve a good balance. For example seeds plus veggies isn't going to have enough protein unless the veggies include a significant amount of beans. The calcium will probably be deficient too. You simply aren't going to get any vitamin D3 or B12 this way because these vitamins aren't available from plant foods.
A study by Donald Brightsmith (he's famous for studying the parrot clay licks in South America) found that a diet of 25% pellets, 25% seed and 50% vegetables and fruits contained too much fat and was deficient in calcium, sodium and iron. It was probably deficient in other things too like protein, but they only tested for certain items.
http://www.bioone.org/doi/abs/10.1647/2011-025R.1 I was pretty shocked at these results. The pellets alone would have covered a significant percentage of the needs, and I would have thought that such a large amount of vegetables would have covered the rest of the mineral needs. But it didn't.
Also grains should be part of the diet well a cockatiels diet anyway.I feed my birds green beans but not all the time.Both my tiels weigh 94 grams.