I like the idea of starting a forum, but how do you get enough people using it? It must be very difficult to get it started.
Although, I will say I wound up putting the forum on a sub domain and installing wordpress in the root directory for better SEO. It's possible to silo a forum but have more control with wordpress in the front driving traffic through pages with good on page optimization, etc.
Probably not best way to do it. Without any experience at all I'd say it's better to install your forum in a sub folder instead of a sub-domain as the authority doesn't get passed between the two.
site.com/forum/ instead of forum.site.com
You may know it's really hard work to get any traffic to the forum and get it grow. Be prepared to spend at least 3 - 5 hours a day just for the forum to get this work.