I agree in principle but I have got away with it. The important step is to put the faster stick in slot #1 but both will only run at the speed of the slower. I frequently come across Crucial scans which mis-read the RAM as faster than that actually installed. This makes me think that (a) Crucial reads the Board and not the RAM and (b) some OEMs put in slower RAM than they should to save a few pounds.
It's generally considered a bad idea to mix RAM of different speeds. You may be lucky, or it may cause unexpected glitches or crashes.
You'll only know if you try but I would be inclined not to.
Then again, some people reckon I'm just an old cynic.