I believe that activation keys are tied to one specific version of Windows. So no, you [probably] can't use an Enterprise key on Pro.

You can choose a 30-day trial - just don't enter the key when you install Windows. It's also possible to dive into a setting somewhere (read: config file) and edit it to give you the maximum of 120 days.

Unless it's a site key, or for a specified number of users, you can use it once (that is, you can only have one version of Windows running on that key at any one time).