I've tried both 9-handed and 6-max a bit. I perform similarly in both (prefer 9-handed slightly, but this is just personal preference). I never heard about kickers not as important in 6-max (I think they are just as important). In fact, I'd play almost identical ranges to a game with 9 players. Maybe slightly wider with 6 than 9, but virtually identical.
I think the biggest difference is probably that more orbits in the same amount of time means that your leaks could be amplified. Let us say that you do not defend your big blind well enough (how much or in what way could be a thread in itself). With 9 players, this isn't as significant as with 6 players because you'll be in the big blind more often with 6 players. Say we play a game with 200 hands. In a 9 player game we will have the big blind roughly 22 times, but in a 6 player game we will have the big blind 33 times!
This would massively amplify any big blind leaks you might have always had, but 9 player games didn't make as obvious. The way I look at it is that your play in the blinds needs to be more accurate than with 9 players and maybe even your play on the button (BTN) needs to be better because you will be in position more often with 6 players (also everything more often

but in position is probably more significant).
The other seats matter little I think. I just treat them as CO, HJ, LJ (in addition to BTN, SB, BB). Maybe you could make an argument that LJ more plays like UTG in 9 player games, but I think most players (at least at lower levels) loosen up way too much in 6 max games. They treat them like Heads-up matches or something and play virtually everything preflop! More players blind stealing (partly because of more orbits and partly because of less players perceived) and getting out of line and I do fine playing my "regular style" I would for 9 player games and exploiting their obvious flaws.
To summarize my opinion, 6 player tables and 9 player tables should use similar ranges. Just be more aware of how to play well from the BTN and blinds because they come up more often with quicker orbits. We don't want to amplify our leaks there. Perhaps widen the range slightly if you like, but I wouldn't widen it nearly as much as some do!