Re: password protect a site
If you password protect or use the Down for Maintenance mode your testers will not be able to test any of the live payment services. If that is okay, I would suggest using the Down for maintenance mode. You can then add those IPs of your testers for access in the Admin> Configuration> Down for Maintenance.
Please do not PM for support issues: a private solution doesn't benefit the community.
Be careful with unsolicited advice via email or PM - Make sure the person you are talking to is a reliable source.