@pluto-
You may have VERY unexpected results when trying to attempt this. Where the files may interchange OK, the CSS code will not, thus it may not display properly at all.
What you should do if you are trying to accomplish a particular layout is pick one theme, and modify it from there. Thus, if you choose joshuaink for example, you can pull the random image function from the thinkdreams theme and use it there. Swapping the pages from different themes will only be frustrating, especially if you're a PHP beginner.
My suggestion? I would choose the joshuaink theme, (and yes, I helped design the thinkdreams theme, but I admire GameDudeX's work on the joshuaink theme) and joshuaink has a lot less "goodies" in there, especially when you are designing a theme as a PHP beginner. You can always "add" to the theme later with the goodies.
Here's an example of a Joshuaink derived theme with "goodies": http://www.thinkdreams.com/zenphoto (which is my current photo gallery). I modified Joshuaink with CSS rounded edges from a nifty Wordpress theme I found called Rounded Blue. It works, does what I want it to, and leaves room for improvements.