When adding a new host, the list of classes that Foreman gives you should only be for the particular environment that you would like to place this host in. When two environments contain classes with the same names, you are unable to differentiate between the classes/environments.

I think one way of doing this is to first ask the user to select the environment, and then based on the user's selection, display the list of classes in that environment. This function should also support a "common" modulepath for classes that are common to multiple modules. I define this in my puppet.conf like:

[fcdev]
manifest = /etc/puppet/manifests/fcdev/manifests/site.pp
modulepath = /etc/puppet/manifests/fcdev/modules:/etc/puppet/manifests/common/modules
pluginsync = false