Can a mix of receptacles and light fixtures be installed on a single circuit?
To directly answer your question about the unused switch in your kitchen, you have to check a couple of things before you install a receptacle in it's place:


Is there a source voltage feed in the box?
If there is only one cable entering the box and both the black and white conductors are connected to the  switch(es), then the voltage feed is coming through the light fixture. In this configuration, there is no neutral and you cannot install a receptacle.
Technically, if the source wiring is not 12/2wg, AWG and controlled by a 20 amp breaker, it does not meet current code for use with a receptacle. 
If you do find that you can install a receptacle, be sure to use a GFCI type.
Determine if the wires on the unused switch are energized and where they go.
If you use the line side (source) wires for your receptacle, be sure to safely terminate the unused switched load side wires in wirenuts.