ModX and Symphony are at different ends of the framework spectrum. Where symphony is a full blow application framework and MODx is a CMS framework.
For the SMF integration there are a few SMF bridge plugins but I am not sure how solid or up-to-date they are.
The easy way to do this is to setup the document tree (modx site structure) with a downloads section and create the subsections for each top level ’type’:
- Downloads
--- Games
--- Engines
--- Tutorials
--- Etc..
Then for the additional classifications you can use tags (actually you can use tags for all the classifications, but it might be easier to manages with top level bins).
The biggest challenge will be the submission process. There are a few snippets that provide a method of submitting content directly into the modx document system. The method that will give you the mods flexibility will be
FDM (it is in perpetual beta but is a solid solution). However, since you code, I would recommend just doing a custom submission form (use the
docApi class to easily add pages). By using the MODx document system you will be able to use standard modx resources for searching and indexing.