What is the make/model of your cpu and motherboard?
Support speed usually means the speed at which the cpu/motherboard combo will boot to get you into the bios where higher ram settings can be made.
3200 speed ram may be as cheap as 2666 and will operate at 2666 just fine.
Yes, the cl will go down with lower speed.
cl vs. speed is not a clear decision. Probably matters little anyway.
The key spec for selecting ram is to buy enough in a dual channel kit form.