Jenkins - How to reserve an executor for (a) specific job(s)

As i understand it Kiddo uses the master for 8 executors. What you can do is to add a new slave which runs on the master, let's call it slave-master. I.e. You will have master with 6 executors that has usage set to utilise as much as possible, and then slave-master which has usage restricted to only the short builds. So on your server you will have two jenkins tasks running, one is the jenkins master it self, and two is the slave-master.

For info on how to connect slaves, go to https://wiki.jenkins-ci.org/display/JENKINS/Distributed+builds

Adding to @StephenKing answer, you also have to specify the label name for each job while configuring it, as shown in the below image: