The wiki is supposed to store information about good, well-known and used in the production of the site software to keep all the praises going to the ones who made it and, of course, to store documentation of the ways the site works.
Currently as the reverse proxy on the production server Caddy is used to make the multi-domain experience.
As the wiki we use Mycorrhiza. Why? Because it is simple to set up, use, store and backup.
For the forum we're planning to add the Freedit as a simple alternative to the commercial and other hard-to-use and support software.
The question about chatting: currently Surdeus sees that Matrix is doing well enough so we can use some implementation of the server and the client in the end for it...