Well, each time the rss.php is read by whatever program/browser/etc, a new page is created, reading the current information of the gallery.
That said, if the program/browser/etc is configured to check a RSS feed for a ttl specification, having that on your page will tell the program/browser/etc at what point it should check for new information instead of using the cached file. Having a ttl of 15 means it should check every 15 minutes (or if it's been longer than 15 minutes since the last check). Is that how often you add new photos to your gallery? If not, maybe you want to use a higher number (like 60, 360 or 1440?) or leave blank. I know I can get a lot of hits just from a few peoples' feedreaders and over time, that adds up.
As far as adding it, you'll either need to uninstall the module, edit the plugins/easy_rss/rss.php and then reinstall it, or directly modify the rss.php in your gallery directory (which means if you have to uninstall it later, and reinstall it, your changes are lost). In both cases, simply add <ttl>15</ttl> after the <description> line and before the <generator> line.
Now, your second part. As far as I know, the rss.php should read from the current album you are looking at and pulling a feed for that album (last, random or a specific album), but it looks like it's not working on mine. I'll look into it and see if I can figure out what's going wrong. If you want to make a permenant change to the entire gallery, you'll need to edit the codebase.php in the plugins/easy_rss folder (search for =lastup for where to change).
Hope this helps!