According to the FAQ you linked, the tile backs are compatible. If you want to restrict yourself to the new art style just to make it look pretty, you certainly can, but you'll be limited to any expansions that get (re)printed with the new style in the future. So if you want expansions now, you pretty much have to ignore the art difference.
Among the older expansions, Inns & Cathedrals is the expansion that actually adds the sixth player's basic meeples, so it's the only expansion that will really satisfy that need. True, it doesn't come with an abbot piece, but you can find a placeholder. (You could also omit the abbots entirely when playing with six players; people have been playing just fine without them for a long time!)
Some other expansions are then compatible with Inns & Cathedrals. For example Traders & Builders adds builder tokens and pigs, and they come in all six colors. Plenty of other expansions don't add anything player-specific at all, so they'll be fine as well. (I won't try to list them all right now; you can look at a list.)
All that said, Carcassonne isn't exactly the best six-player game. You can end up with a lot of downtime between your turns unless everyone plays quickly. Each player also doesn't get to play very many tiles, which means they don't place very many meeples, they don't score very many times, and can't be very efficient.
The base game including the river is 84 tiles; if you get say Inns & Cathedrals (18 tiles) and Traders & Builders (24 tiles) you're up to 126, giving each player 21 tiles to play. Even if you play a meeple on every single tile the game will be 1/3 over before you get them all down. If you want to complete your cities, you'll probably be stuck making a lot of small ones. And it feels pretty random; with five other players taking a turn before you get to go again, you have very little control over the things you start to build. Rare tile shapes are basically impossible to get. (Good luck with the four-sided city tiles!)
If you really want to play Carcassonne with six players and still have a serious game, I'd suggest playing with people who are familiar with the game and can play pretty quickly, and buying a lot more extra tiles. That might mean multiple copies of one of the large expansions or even the base game; you'd be buying them for the tiles, not for the extra mechanics.