Further, would it be a good idea to not allow changes or additions to these mid-game? --Pallando
Certainly. All players should be clear from the start what all the rules are of the game they're playing. When teaching someone a game for the first time, there will often be rules only mentioned partway through, either through a desire to simplify initial learning or through forgetfulness; but in general the strongly desired state is for everyone playing to know what's possible in the game they're playing. --AC
A couple of notes on specific rules you mentioned here:
Trading between players may happen during either player's turn, or with the permission of the player whose turn it is. (Such trades are considered to be all goods passing through the player whose turn it is.)
More than one progress card may be played per turn (This is a standard rule. We abide by the printed rules on when progress cards may be played: only on your turn, and not if you have played a card with that name already this turn.)
The Crane (cost of improvement reduction card) may only be used on a single improvement.
I'm sure this isn't how we used to play. Can anyone find the official rulings on the Crane, as this isn't a house rule but simply a ruling to establish what to do in the case of uncertainty? --AC
Trading with the bank, which means: 4-1 (bank), 3-1 (any port), 2-1 (special port or card) may produce commodities as well as resources. (This isn't a house rule but a part of the normal printed rules)