Can I mix RAM sticks with different speeds?

Yes, you can mix them. The motherboard will automatically downclock both sticks to the speed of the slowest one (DDR4-2400). The performance hit is real but usually not noticeable in everyday use. One catch: mixing different kits can cause stability issues, so test with MemTest86 if keeping long-term. If buying new, just get matching 3200 sticks.