Before starting on the actual rules in the actual books, and what I'll allow and what isn't allowable, let me give some general rules before we even start.

• Rule number One: Powergamers are not welcome! 3.5 and Pathfinder 1e were famous as munchkin friendly combo builders for character optimization. I always played them as if the options were roleplaying options that had to interest me from a roleplaying perspective rather than a power-game perspective, so I rarely to never saw any of the problems that lots of other people reported about the game. I had a good group in that regard, and I do again now, mostly. People who want to squeeze out cheap, cheesy mechanical advantages aren't very fun to play with, and these rules interact badly with that type of player.

• I tend to be a little bit handwavey about rulings in actual play, and don't really like the obligatory grid-based combat. I strongly suggest not building characters around the concept of tactical advantages, because that will come up less in my game than you might expect. Build characters that are fun to roleplay. Because I run with a dark fantasy paradigm that's maybe more like Call of Cthulhu than D&D, don't simply count on combat being quite as prevalent or prominent as your typical D&D game, don't build characters that are only good at combat, and don't think of combat as the go-to solution to all of your problems.

• Elephant In the Room house-rule is in effect. I may copy the actual rules here on my site, but for now, just follow the link to the creator's site. I need to review along with 3.5 and make sure that it aligns as well as it does with Pathfinder 1e.

• Same for E6. E6 was actually developed for 3.5, but it works equally well for Pathfinder 1e, which is clearly a minor advancement of the same family of systems. 

• All races except human will likely be homebrewed, but I will review the race rules that I have, and post them as soon as they are available.

• Classes will all be taken from Pathfinder 1e rather than 3.5 Also from Pathfinder will be the slightly simplified and consolidated skill list and the CMB/CMD rules.

• No spellcasting (or psionics) fullcaster class can be taken at first level. You can multiclass without penalty into it at 2nd level, between that and whatever class you take at 1st level. This will limit the availability of magic even more than simply using E6 by itself does, which is crucial to the low fantasy sword & sorcery dark fantasy feel that I want to engender. 

• Speaking of base classes, bards and monks are not allowed at all. The monk simply doesn't fit the mileu, and given that it's not the early 70s anymore where Bruce Lee and David Caradine's Kung Fu show are super timely, it fits even less than they did when it first debuted in 1975 in the Blackmoor supplement. The bard, on the other hand, I just don't like, and I distrust anyone who wants to play one for... I think probably obvious reasons. "Halfcaster" classes like the paladin or ranger can be played as normal, but cleric, sorcerer, wizard, druid, etc. can't be taken until 2nd level.

• Speaking of clerics, the D&D paradigm isn't how this works. Most actual clergy are simply experts, or at most adepts of the NPC class variety. Clerics represent militant mystery cult fanatics. There aren't friendly clerics taking the role of a small town preacher like in D&D; clerics are usually mistrusted because they're seen as wild-eyed zealots. The roleplaying implications for anyone playing one should be obvious. 

• If clerics are religious mystery cult fanatics, druids are even moreso. Druids aren't like kind of friendly hippies, they're more like dangerous and insane wilderness cultists, and they're seen as even more untrustworthy and dangerous and unwelcome than clerics. While clerics may be given the cold shoulder by most normal NPCs unless they play it low key, druids are likely to be lynched. Frankly, wizards and sorcerers (and when I get around to adding inquisitors, witches, oracles, etc.) are all seen as dangerous, untrustworthy and unwelcome at best, and as dangerous and hostile at worst. 

• Incantations can be used to introduce higher level spells at GM's discretion, but because they're incantations or rituals, they're not really combat spells, and they don't serve exactly the same function. 

• A few third party classes I'm already aware of me allowing: noble and freebooter from Freeport: The City of Adventure and the soulknife from Ultimate Psionic. I'll eventually reproduce the rules here using the auspices of the OGL, but for now, I'll just refer you to the original source.

• At GM's discretion some higher level class abilities, or more class abilities that can be chosen from a menu, such as the rogue talents, etc. regardless of level can be taken as if they were feats under the above level 6 rules for E6. 

• Archetypes are mostly in (I haven't read many of them yet in my re-read, but I'll make a list of any specifically disallowed as I come across them.) Because archetypes is a better mechanic than prestige classes for the same general idea, prestige classes are out. You can't use them under any circumstances.

• The Madness rules from Freeport: City of Adventure will be used. When I get to Horror Adventures I'll reevaluate which of the two I prefer. I might even adapt the Sandy Peterson Cthulhu Mythos dread rules, since they're 5e, which is sufficiently d20-like that it'll work. I'm not a huge fan of the Unearthed Arcana Sanity rules, so at the very least, the Freeport rules are an improvement on them.

Honestly, I'll probably drag this out. I'm feeling charitable towards the broader d20 system in general right now because I'm re-reading so much of this, but realistically I'd rather not use that system as opposed to my own sort of OSR adjacent rules light system.