Can I use a multiplexer to connect a small capacitor to a 555 timer?
You should read the datasheet of the analog multiplexer and look for capacitance values, especially input capacitance. Those unwanted and unavoidable capacitances will influence the frequency of the 555 timer, especially if the capacitor connected to the 555 is very small, about a few pF. The unwanted capacitances are not constant and will influence the stability of the generated frequency. For good stability, the smallest capacitor used should be much larger than the unwanted capacitances of the multiplexer.