Stranded wire in light switch
The screw terminals on standard switches are easy to wire with solid wire, more difficult with stranded.  

It's easy to make a loop in solid wire that fits nicely under the screw head.  With stranded you have to wrap the wire around the terminal and tighen it without it coming out from under the screw head.  

Me?  I'd hunt up a scrap of solid for the pigtails if at all possible, but I would not make a trip to the store if all I had was stranded.  

The push in (aka "backstab") terminals on standard inexpensive switches are for solid wire only, and most agree that they are marginal at best with solid wire.  With stranded wire, totally unacceptable.  

However...

This switch does not have the usual terminals

The terminals on this switch accept the wire like a backstab, but they are not a simple push in;  there's a pressure plate inside the switch that applies pressure to the wire when you tighten the screw, similar to the mechanism on most breakers.  It's easier to use and makes a better connection than a regular screw terminal with either solid or stranded wire.  



In fact, I'd go as far as to say this type of terminal makes a better connection with stranded wire than with solid.  

Now at the other end of the pigtail,

where you're splicing to the existing solid wires - it can be tricky to get a really good splice with a mix of solid and stranded, especially if you try to pre-twist the wires before putting on a wire nut.  With push-in type connectors, it's easy to mix solid and stranded.