Can I use DDR3 1600MHz RAM in a Mac instead of 1067MHz?
You could add the 1600 MHz memory for your MacBook Pro even though it's designed for a slower memory (1066 MHz) - it ought to work, but you'd be better off buying from a place that accepts returns. Here's some relevant information from Crucial on memory speeds (emphasis mine):


  Memory is designed to be backward-compatible, so generally speaking, you can safely add faster memory to a computer that was designed to run slower memory. However, your system will operate at the speed of the slowest memory module.


In "DDR3-1600 PC3-12800"


"DDR3" is the type (and must be the same as what your computer has)
"1600" is the data transfer rate per second specified in millions (the technical term is "clock frequency") - "1600" here means it supports up to 1600 million transfers per second - in your case, it will be slowed down to 1066 million transfers per second since that's what your Mac is designed for
"PC3-12800" is the name used in the memory industry - 12800 indicates a peak transfer rate of 12800 MB/s (of course, the actual performance in your case would be brought down to 8533 MB/s, matching PC3-8500).