As already mentioned, the input doesn’t matter to Unity, which uses DXT for final images no matter what. You could have 1 bit per pixel or 1,048,576 bits per pixel and the image will always be the same size when Unity publishes the game. Similarly, you can save the image as raw uncompressed data, or JPEG the heck out of it, and it will make no difference to Unity…so you might as well stick with lossless formats, because you won’t gain a thing with smaller input files aside from loss in quality. So, 8 bit, 16 bit, or 32 bit makes no difference. In other words, don’t bother trying to reduce the color palette; you’re unlikely to gain much of anything. 
It’s a moot point, but just for the sake of info, you can’t have more than 256 colors for indexed color regardless. 256 is the largest number you can store in a byte. Indexed color is named thusly because it uses an index for colors rather than specifying them directly. Color #47 (or whatever) could be red, purple, green, or whatever you want…all 256 colors in the index are stored in a list which is then referred to when drawing the image. So you can pick whichever 256 colors (or fewer) are best suited for the image at hand. A picture with lots of blue in it would naturally devote most of those 256 colors to various shades of blue.
If you want to go beyond 256 indexed colors, then you’d have to use more than one byte per pixel. So, for two bytes per pixel, you have a maximum of 2^16 possibilities instead of 2^8, which is 65,536…that would be one heck of an index! You’d need 192K per image just to store the data for the index…compared to 256 colors which only takes .75K for the index. Obviously that’s not very efficient, so as long as you’re using 2 bytes per pixel anyway, you might as well specify the colors directly. “Thousands of colors” on the Mac uses 5 bits for each of the red, green, and blue values, and the last bit is wasted I suppose, giving you 32,768 colors. 16-bit color uses 5 for red, 5 for blue, and 6 for green (because the human eye is more sensitive to green), which gives you 65,536 colors.
More than you ever wanted to know.
It all ends up as DXT1 or DXT4/5 anyway when Unity is done with it (though your source images are never touched), so, like I said, a moot point…
–Eric