Atlassian Cloud support limit their support to addons that they are trained on and willing to support. They don't have the resources to be able to guarantee stability and support on a system where users can install everything they feel like. (There's a balance here between cost and flexibility too - the more you allow, the more expensive it's going to be to support it)
The "Confiforms" plugin, specifically, I can tell you that is not available for Cloud instances.
Any third party plugin is not supported by Atlassian, however, most of the vendors of these plugins offer support for it.
You can confirm if a plugin is whether supported or not in the plugin page on market place, through the "Supported" or "Unsupported" icon.
As you're looking to use Confluence Cloud for a customer facing website I thought you may be interested in Instant Websites for Confluence Cloud. It allows you to take a Confluence space and publish that to a fast, mobile responsive website hosted on Amazon Web Services.
Here is an example of our product documentation.
If you have any questions I'm happy to hop on the phone and explore further.
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