A quick and easy tweak to the game rules so that it accommodates 5 or 6 players is simply have every player start with a hand of four "1 gold" cards instead of five. The result is an insignificantly higher chance of getting more units in your starting hand on round one and nothing more.
I would also suggest using six neutral city cards for a 5-player game and seven neutral city cards for a 6-player game.
For other setup rules that scale with players, such gold cards and neutral units, playing the 4-player setup with 6 players actually isn't altogether that bad. Plenty of cards still end up being destroyed, sent back to the middle area and made available to the other players. The various card piles will end up depleting faster on a round-by-round basis, but it happens at such a small difference compared with the 4-player game that all it ends up doing is cause deeper player tension with racing to get the cards and destroying the players who get them first. You might even say it just makes the game slightly more competitive.
I would also reccommend treating players 5 and 6 as if they're also "player 4" for purposes of card effects. Fortunately the game was designed to occur turn-by-turn for each player, so each player either suffers or benefits from every event card effect equally, regardless of how many players there are. However, "player 4" tends to receive either a very large bonus or a very large penalty from certain event cards and scaling the effects higher than "player 4" levels can cause some more noticeably balance-breaking results.
So in summary:
-Give each player 4 starting gold cards instead of 5
-Use 6 or 7 cities for a 5 or 6 player game
-Players 5 and 6 are treated as player 4 when awarding penalties or bonuses from card effects.
To answer your question about Cataclysm and Dragonlords specifically, you lucked out with the way turns and rounds work - Both scenarios work just fine with 5 or 6 players, so long as players 5 and 6 function no differently than the other players do.
Give it a try and see if it works out!