[i]Can I use the same db for wordpress and zenphoto?[/i]
Yes, you can use one database as long as you use different prefixes for the tables.
(Zenphoto uses zp_ and WordPress uses wp_)
[i]Is this the right way to do it?[/i]
Should not be a problem.
[i]500 Internal Server Error[/i]
It may be a problem with the .htaccess file -- some servers don't support them. Try removing it and turning off mod_rewrite to see if that helps.
Actually this last answer was copied from an answer somewhere else on this forum.
A forum search might really help ;-)