There is no staging environment for Cloud instances but, generally, most add-ons are going to be completely hassle free, particularly if they're from a well known developer. If something goes wrong, we can rectify that and of course if something goes very wrong we automatically backup your Cloud instance every day.
If the issue is one of being able to internally test and configure an add-on prior to making it available to your users then, depending on the add-on, you can usually do this with the permissions system so that the add-on is only available to certain users until it is ready.
In terms of keeping the site offline, sure. You can use the "Board active" option in the admin control panel to close the forum to anyone except administrators, while displaying some sort of maintenance/coming soon message to other visitors.