Not ideal but usually OK, it will run at the lowest speed of the two (assuming the CPU could run the higher speed), same for CAS but in this case at the higher of the two, note that the reported CAS is for PC-10600, it will likely be the same at PC-8500.
Depending on the server more ranks can run slower, especially with 2DPC, so use minimum number of ranks available for the DIMM size you want.
This, like mentioned ideally all DIMMs should be identical.