How to delete simulators below iPhone 8
In Xcode, 


Go to Window -&gt; Devices and Simulators or Shift + Command + 2.
Click on Simulators at the top of the window.
On the left side you'll see the list of all the simulators installed on your system.  When you select one, you'll see the details of that simulator (OS version) on the top right.
Select the one you want to remove and press Delete.  Confirm your request on the confirmation dialog.  You can only delete one at a time so it will take some time if you want to delete many of them.


Xcode will delete the simulator.  After this, I usually restart Xcode but I don't know if that's required to clear out the hard drive space.

Regarding your PS, there's no correlation between deleting an old simulator and your app running on a device.  The only thing you lose is seeing how the app would look on a device you don't personally own.  Only you can judge the value of that.