I don't see any reasons why it wouldn't be possible to have Standard and Enterprise on the same server. I haven't tried just that, but I HAVE tried installing Express on a machine with Developer installed, which should be more or less the same thing.
However, I don't understand why you would WANT to install a Standard Edition instance if you already have Enterprise. Licensing wise, you can have any number of instances using the same license, if they are installed on the same machine.
↧