I have a test folder (seperate from my normal xlights folder) with a Null of 100000 channels. Then I can add whatever I like in Layout for testing.
This way there's no chance of mucking up existing controllers, channels, layout, sequences etc in your normal folder; and it's heaps easier now with the ability to temporarily change folders.
If you want to see what a prop looks like on your current layout, you could copy the necessary files from your current folder to the new test folder and then add the null for test props.