Hi,
I was wondering,if I were to use a 16MHz crystal with a 18pF load capacitance with a ATMega328 would there be any issues? If not could I still use 22pF caps or would I need to use a different value? Thanks.
No Issues there. I made a DIY Uno using an 18pF crystal - you just have to use the correct load capacitors - an 18pF cap from each pin of the crystal to ground worked fine for the crystal I had.
The feedback capacitors are effectively in series, so something like 33 pF is needed for an 18 pF crystal.