Clock cap value critical?

All I can say is that as long as it works, there's no problem.

I guess a too big cap will smooth the sq. wave too much and a too small will not filter a lot of voltage ripples.

I often see 22 and 33 pF so I guess 22 would work good. This is not the best mod to do tho. IMO Building a XO clock circuit like Kwak or Guido clock is a better option. If your goal is to get low jitter, then it's the way to go.
 
I guess a too big cap will smooth the sq. wave too much and a too small will not filter a lot of voltage ripples.

The caps are a part of the internal chip pierce oscillator. Changing the parts have a small influence on the frequency (some 10th ppm) and changes the feedback factor, which can cause start up problems. One value up or down (27p or 18pF instead of 22p) shouldn't led into trouble but I wouldn't go further.