You got it right. Use any name for the “Save matching device list to variable” and then use that name between { } in the SMS message.
When saving the info to a variable you can use @ to make that variable global. This means other pistons will have access to it as well. You don’t have to, unless you need that info in other pistons. You won’t be able to set any variable starting with $ as those are reserved system variables - they change on their own 
The example you showed should work.