In general, 3.5 and Pathfinder are extremely similar systems. For the most part, rule-changes will probably just work as-is, or work about as well as they did in 3.5. I’ll discuss various aspects of the systems below, with bold used to indicate places where more care is required.
Changes that will strongly affect the application of 3.5-based rule-changes
The only big, sweeping changes that require real adjustment are combat maneuvers and skills.
Combat maneuvers use CMB/CMD rather than the various opposed rules used in 3.5. Generally speaking, this should make it easier to mess with those systems, but some of the math from 3.5-based rules-changes may be off. These will probably be the trickiest to deal with.
Skills usually just requires the adjustment of the number of skill ranks necessary for things, and possibly translation from 3.5 skills to Pathfinder’s reduced skill list. Aside from Concentration and Use Rope, there is a direct mapping from 3.5 skills to Pathfinder skills, so this is usually pretty easy.
Notable changes, that nevertheless probably can allow rule-changes as-is
The feat progression changed, but that rarely matters much in terms of how well a given rule change will work. The feats themselves changed, and several were split into separate feats where they were previously a single feat, but unless a rule change relies on a specific feat directly, that won’t matter.
Similarly, most classes changed a little, but it’s mostly just the addition of a few minor features; nothing that’s likely to impact whether or not a given rule change is a good idea. The barbarian and bard changed the most, so rule-changes from 3.5 with respect to those classes are relatively likely to be awkward, but I’d guess that for just about every other class, a rule-change can be used as-is.
Then there are the spell changes, but those weren’t very large. The change to magic to change form (polymorph, Wild Shape, etc.) are the biggest changes, so rules-changes that hit those specifically may be problematic, but that’s about it.
The specific changes listed
None of these are going to interact with Pathfinder in ways noticeably different from how they interact with 3.5.
Feat every level
Even with Pathfinder’s accelerated feat progression, feats are still often weak, and very often used as unnecessary “taxes” for interesting options. The fact that Pathfinder actually nerfed quite a few feats for several of the most feat-dependent classes only exacerbates the issue.
I have played in many games where I had a very large number of feats; it allowed more interesting and unusual things to be done, but never caused problems. But then, no one involved was looking to cause problems, they were looking to do unusual things that the usual limits on feats prevented them from doing. As with anything else, it relies on a trustworthy group.
Combat feats are more powerful when taken as Fighter bonus feats
A great idea; there are just way too many feats to go through them all and actually determine what the fighter bonus would be. Probably easiest to run this and tell fighters that you’ll make up extras for the particular feats they take, rather than trying to do them all.
Double feats that take both a normal feat pick and a Fighter bonus feat pick to select
I don’t love the idea, but if it justified some more-potent abilities for mundane types, it’s worth considering. Mundane types like the fighter suffer immensely in both 3.5 and Pathfinder, after all.
But note that there are other mundane and low-magic types beyond the fighter. Give too many goodies to the fighter, and it’s going to be very difficult to justify barbarians, paladins, and rangers. Especially considering the nerfs those classes received in Pathfinder to begin with.
Fighting styles that give bonuses for taking related feats
Official 3.5 did this, but not much. A fairly-decent idea. Again, specifics matter far too much to say what is or isn’t going to work well. But the differences between Pathfinder and 3.5 on this are minimal, so if it works in 3.5 it should not be a problem in Pathfinder.
Twenty spell levels
If 20th-level spells are equivalent to current 9th-level spells, and this releveling just represents a smoothing-out of the system, that’s fine.
If the 9th-level spells are as they are now, and there are 11 more spell levels above that, no, absolutely not. 9th-level spells are already more than capable of breaking the entire campaign into itty bitty pieces. Anything that’s that much better than 9th-level spells would be basically unplayable; you’d be able to do anything, any time.
A huge spell rework
Would need more details, but if it’s “huge,” the only likely way you’ll get an answer is as a separate question, asking for people’s experiences with those rules, since to try to go over an entire spell system rule by rule would be a lot much for an SE question.
