The quote to govern is to populate means that in order to have an effective government, you need to have the right persons with you.
This quote tells the reader how Mexico wanted to populate the area but in order to do so they needed the right people.
The right people here would mean the people that are Anglos and Catholic. To them these were the right people that would make them not to repeat the mistakes of the Spanish.
Although you didn't include the picture, one of the reasons why Spanish Texas almost failed was because of hostile native Americans that lived there. These would be the wrong people that Alberdi was trying to make reference to.
