i often heard about special procedures when adding ps to your shoal. i heard all kind of things but the one that came up the most was to make a big water change or change physical space (if you have plants or drift wood) so you'll change their environment and they won't feel as territorial and accept better newcomers.
i think there is an age factor also. if your ps were all the size of the ones you showed on the pics, they weren't too old yet so that probably helped them shoal together faster. maybe a water change would have been good for your new eyemissing fish... but never experienced anything like this, just heard of...