Your only real concern with doing E8 as compared to E6 is that everyone gets access to 4th level spells. Really I see this as the deciding factor.
1) Yes, there isnt anything that has changed to make pathfinder have an issue with E6/7/8 rules. In fact if you search there are several discussions on things like capstone feats and which of the 3 E's work best. Personally I think E7 is the best choice for pathfinder.
2) The ones you mention will work just fine with relatively minor asdjustments. Most of them have a handful of threads here with suggestions on how to adjust them, but for the most part they are ready to go.
As far as using the 3.5 creation rules with Pathfinder rules, I dont know how well that would work. Like I said, most of the 3.5 extra base classes need very little work, so if you are going to use pathfinder use pathfinder classes. Most 3.x options/spells are compatable with minimal work. My group still uses the complete series, the spell compendium, the magic item compendium, and the PHBII along side our pathfinder books and our pathfinder 3rd party material.
Sorry I should have more appropriately answered your specific questions.
1. Should work fine with only minor work (namely if you want to work some feats up, etc.)
2. Depending on the specific class some will be stronger than others, but as Kolokotroni said, there have been several conversions made on the forums. From a roleplay perspective there shouldn't obviously be a problem.
I'm actually planning for my next game to be E6 myself. I think it's a great system and a great area of play.
IMO E8 is awesome, as 8th is a nice plateau to rest a campaign on.
one of the things i've done is actually run an E6 and then for the end parts of the campaign, just jumped the characters to 8th lvl to signify the ensuing epicness
I have to throw in my vote for E7. Allows limited 4th level spell casting; give full BAB that extra attack; gives a lot of classes a nice cap like DR, more SA damage, etc. You can make quite a few of the 8th level abilities into feats, and, in fact, quite a few already are. What does a rogue get at 8th? A talent. There's already a feat for getting an extra talent.
Another nice thing about capping at 7th is that it eliminates +Stat items, which require CL 8th to create. No +AC item can be greater than +2, and no weapon have a more than +2, either. To create a +3 item, caster level has to be 3X the bonus, so CL 9th. Bracers of Armor are a little different, but those cap out at +3 anyway. Flaming, frost, etc are out the door too.
So E7 cuts out a lot that E8 doesn't, but allows you make most of 8th level abilities available through feat. It cuts down on the Christmas Tree Effect, gets rid of what turns a lot of games into equipment/item management/numbers crunching.
Why 7 over 6? 6th level is like 8th in a lot of ways. What most classes get at 6th is something that you can get with a feat anyway. There's very little, capstone-wise, at 6th. It does limit casting even further, but I feel with judicious use of GM oversight, 4th level casting isn't a problem.
One other thing to consider if you're going for 7 or 8 is that XP values and level progression have been inflated in PF, so the old 3.5 E6 XP/Feat level of 5,000 needs to be revised. You'll barely have time to play around with a new feat before you grab another. 5,000 xp will come quick, especially at 8th level. If you kept everything relative, from the rate you would gain 5,000 XP in 3.5 and converted the numbers to PF, at E8 you'd be looking at around 20,000 XP to earn a new feat.
I love running my E6 game in Pathfinder. It plays like Pathfinder was made for it.
Mostly that was because pathfinder did a bunch of things to improve low level play and to give more interesting class abilities so you arent waiting multiple levels for your next thing. But I agree that Pathfinder is very good for the E6 concept.
What about giving the players Favored Class bonuses and a feat?
That way they will gain a marginally survivabilty bonus, an extra skill point or could build up thay alternate favorad class bonus from the APG.
Its to little for impacting inmmediatly, and would take a lot of play time for being noticeable.
Humbly,
Yawar
@ the OP on "3.5 vs PF" in E6:
The tricky thing to navigate is what to do with Cleric Domains and Wizard Schools. Some Clerics get their 2nd domain ability at 6th, some get it at 8th. A similar problem exists on the Wizardly side.
Some people also argue that Sorcerers should get their 9th level Bloodline ability, or else they never develop the "feel" of the bloodline.