I am pretty sure the answer is yes, they are supported. Best thing to do is check each website and see what the specs are. Then come back to Dreamhost and look at the specs, I beleive you will find a match.
Most forums will work as long as there is php and mysql. Dreamhost has both. Mysql I don't beleive is the latest version, but I could be wrong. Besides Dreamhost allows you to install them for yourself if you need to.
The only problem that I find with Dreamhost, is because the servers are shared, you can not run programs that run in the background or are daemons, no live video streams, no irc's, etc. Things that are resource intensive. Forums luckly do not run in that category. You can however stream recorded video that is stored on the server.
Well, I hope that answers some of your questions. You may want to contact Dreamhost as they are the final authority.