I realize that this is a very old post, ans seeing that the user who posted this well over three years ago has likely given up on receiving any type of answer some time ago, but I would like to approach this question so that it might be of some use to others who might stumble upon this entry in a search.
To tackle the first question. Using the 'umask' command the default file permission for detailed information you can look at the man page for umask or take a look at:
http://linux.about.com/library/bl/op...ewbie3.2.6.htm
As far as the second portion of the question, I am not sure of the behavior that existed with RH5.2, but in Fedora Core 1 my experience shows that when a file that is already opened is allowed to be opened by another user, but the subsequent file openings are 'read only'. If I recall correctly, the behavior for RedHat was similar going back to at least RH8. I assume that other modern distributions behave in a similar manner, which should hopefully address this concern.