The single source of truth meta rule does not mean that homebrew content is not allowed!
The opposite is true, players are encouraged to design and propose—or copy "inspiration" from elsewhere, adjust, and propose—their own feats, spells, classes, and magic items.
Since the setting strongly diverges from the 'default' Pathfinder setting, feel free to consult with your GM on the availability and setting-appropriateness of concepts.
Some guidelines have been written out to assist both when adjusting existing content or designing new things:
Conversion
The conversion of spells, feats, classes, and other rule-related things for the Maalum setting will mostly be done in a 'see what players are interested in, and convert that' fashion.
Since there is a lot of content that could be ported, the GM will adhere to this 'player driven design' plan so much so that only things the players are interested in are likely to end up on the Rules wiki.
This means that some initiative lies with the players to prod the GM with questions and suggestions of things they would like to see available.
Core Rulebook: Most of the skills and feats from the Core Rulebook can be ported directly, some spells will need adjustments, and most magic items will most likely need some rewriting to fit the campaign. Classes are best used as inspiration, as a choice has been made to use no Core base classes (with the exception of Fighter-like classes).
Occult Adventures: Most feats and spells can be ported directly, taking into account the adjustment guidelines for spells. All classes (with the exception of Mesmerist) have already been ported. Mesmerist has been explicitly left out, but might offer good inspiration for social classes.
Mythic Adventures: Most spells, and feats can be ported almost directly. Some might have to be adjusted as per guidelines. Note that mythic paths and mythic abilities are not yet fully ported, and adjustment guidelines will eventually appear.
Inspiration
Note that the unused parts of the Source Books (classes, spells, feats, and any mechanic that is not used) can offer great inspiration for the rules-wise implementation homebrew spells, feats, classes, and other custom things.
The same holds true for other books. In particular, the following source books are of great use for their class feature alternatives described in archetypes and classes:
- Advanced Players Guide
- Advanced Class Guide