Most people don't mind that you use their subs in their scripts. There are others that don't mind, but you must leave their subs in tact and in separate files that are called so they can still be version controlled by the original author (EN is a good example of this practice)
So, if the author doesn't prevent you from using their subs, then it's OK to leave a reference/link/thanks to the original work in your header and the subs themselves you can put in your code somewhere in their entirety. If you have modified them in some way, then it's just good practice to document your changes.
Now if you are modifying an entire script, then in order to post here, you need to get the original authors permission. If the author has an account here, then they can PM me their consent, otherwise they can PM me at EUO (Miffy).
Hope this all is reasonable enough for ya! We try hard to protect people's work, but sometimes things slip through the cracks.