how close in value should the capacitors for the crystal be. If I found a pair of 18pF; would these be ok? or maybe values that were larger instead. 22pF are hard to come by.
Those 18pf caps will work just fine. Changing the cap size slightly will just result in the crystal frequency changing slightly, not enough to effect anything as the typical microprocessor crystal is already only a +/- 50ppm stability device which would mean +/- 800hz for a 16Mhz crystal. Your smaller caps might even move it closer to 16 Mhz for your specific crystal!