init_startup should only really be used when you have things which need to be done when the script is
just about to start (i.e. when you want to cache extra templates/fetch extra data from the datastore) otherwise your best to stick with global_start as thats when most things are loaded and ready to go. If you just testing however use one of the less used pages, personally I use one of the hooks in the FAQ for testing, when I have what I want working then I would move it over to the most suitable hook.
Good luck