One of the reasons we've gotten support from the phpbb community is that we port the author's styles "as is", with all credits where they were originally placed.

We have added the author info to the the footer of the site as well, so removing the same info from the forum footer shouldn't be a problem as long as you leave the credits in the site's footer intact. I can't speak for the style authors though, so if you recieve an email from one of them, its up to you to act accordingly.

Removing phpbb's credit is frowned upon, so it would be wise to leave that info in the forum's footer.