I'm not sure if this is helpful, but this is how I have printed multiple different images in iPhoto;
-Select (command + click or click + drag) the images you wish to print and click print
-Select 'standard' as the theme
-Over on the bottom right of the printing box you have an option 'paper size', so select your paper size.
-below that there is an option 'print size', there you can select 6x4 or whatever you want, this is for your actual photo size.
-iPhoto will automatically fit as many photos of that size as it can onto your paper size.
-when you click 'print' again, it will take you to your print/quality settings where you can adjust them before you really 'print'.
HTH