As for the overwriting everything. Basically yes they do. Especially if you want to overwrite things like the CSS and/or "intercept" some hooks or call backs. Except maybe for some core functions. Which you can actually create your own sub-core functions something like a framework and call your custom codes from it.

When you do a ossnregistercallback() to initialize your component there is a default priority of 200. If you want your component to initialize before the other component you can specify the priority like 100.

For example: ossn_register_callback("ossn", "init", "my_component",100);

This loads/initializes the "my_component" before the other components with the default priority. And if you want your component to load/initialize last, you can use a value like 999.