Can I use a 3600 MHz RAM kit with a CPU that only supports 3200 MHz?

Yes. The kit will run at the default JEDEC speed (usually 2133 or 2400 MHz) without any input. To get 3200 MHz, you have to enable XMP/DOCP in BIOS and set the frequency to 3200. The memory controller handles 3200 just fine — it’s the rated speed, not a hard cap. The 3600 kit is usually cheaper per gig, so it’s a no-lose move.