[Puppet Users] Referencing a variable from one class in another

Yes, they can. That's the main selling point for the "include class"
syntax. And you are right, you can't use the class {...} syntax more than
once or you get a duplicate definition error.

However, let me warn you against going overboard with having classes
include other classes from other modules. It can be annoying to track down
where resources coming from for any given node if you've got cross module
inclusion: kibana includes httpd includes mod_ssl includes openssl includes
somethingelse includes ... How did this get on here?

A cleaner way might be to declare cross module relationships using the
Arrow operators:

class kibana::apache {
Class[apache::params] -> Class[kibana::apache]
...
}

And then you make a house rule to have all your classes instantiated in
your node definitions:

node woof {
class kibana
class apache::params
}

If apache::params is missing, you'll get an error saying so. It also fits
rather nicely into an ENC if you want to go in that direction now / later.

-Luke