If all RAM modules have a common set of timings and bus speed that they can run at, you can usually mix them. Of course, nobody will guarantee it to work, but at the same time, nobody can guarantee that it won't, either.
The best advice has already been given which is to buy a single kit that meets your needs. It's easy to understand that's not always a practical route, especially with high RAM prices.
Beyond that, it's going to depend on your particular situation, whether higher bandwidth or greater capacity is going to best suit your usage.
Personally, I usually lean more toward capacity than speed, as when a system runs low on RAM capacity, it tends to be far slower than even the slowest RAM modules available.
Since you have the capacity to run 4 RAM modules, if the opportunity is available to you to attempt the mixed configuration, I don't see any harm in trying it, but I certainly wouldn't purchase new modules just to try them.
You will probably see best results from keeping the 1333 MHz modules and the 1600 MHz modules on separate memory controller channels. I'm going to speculate that means one 1333 MHz module in a yellow slot and one in a black slot, but I'm not sure which ones. The same would apply for the 1600 MHz modules. The manual may point out which memory slots correspond to which memory controller.