There is no way to get Pulse itself to run a subset of your tests, because it doesn't really understand how the tests are run. You would need to implement some extra logic in your build scripts to do this part. Pulse may be able to help feed in a parameter (or environment variable) to the build so your script knows which selection of tests to run, though. For example, you could have a property called fast.test defined on your project and set to true (passed to your build either as a command line argument or environment variable). Then you could have a nightly cron trigger that overrides this property in Pulse with value false, so that all tests are run for the nightly build.
Hope that helps,