If I choose aerospace engineering in an NIT, will I able to get into ISRO?

If you too are absorbed by ISRO, under the new placement, the bond period will be of 3 years, which actually is an advantage. A degree in aerospace and a work experience of 3 years in a core aerospace organisation like ISRO makes a really good profile for higher studies. However, keep in mind that,

If your marks are good and if you have cleared GATE with good marks, you can get yourself into ISRO.