Successful collaboration between or among nonprofit organizations must be built on a foundation of

The answer is: Trust

Non-profit organizations were established to fight for a common good rather than for profit.

This mean that when more than one nonprofit organizations work together, they tend to skipped various costly legal formalities that usually made to protect organization's interest and not lose its profit. Because they overlook this, they need to build a foundation of trust in their relationship.

an informal structure in a bureaucracy would be something like so-called "office politics". where there's a formal structure set up for the organization, but then an informal subculture as well that has a way of organizing the bureaucracy further. this can be positive because it sets up an added degree of organization, albeit informal, within the bureaucracy.