I've run a couple sessions and really like the game, but I have a couple questions.
First, how exactly does catching a beat work outside of a conflict? I get that in a conflict you can use your leftover dice to take another action, but when you don't have Action Dice/Defense Dice, can you just make another roll with whatever dice you would get normally?
Catching a beat doesn't let you gain any additional dice to use for your bonus action. If you have any dice left over you can use those, or you can do an action that doesn't require a roll (taking an extra step for example). If there aren't any actions that your character can reasonably do without any dice, then I would recommend using those 3+ nudges to give the members of your crew a group benefit (+1D to everyone's action the next beat, restore a point of everyone's Resources/influence/spirit, temporarily boost everyone's reputation or willpower by 1 point, etc).
Would it be reasonable to allow a weak-powered character to take one of the stunts from the Terris or Skaa book instead of an extra trait? They cost the same amount of advancements, so if that isn't official anywhere I might add it as a house rule.
Absolutely! Although it's not explicitly mentioned anywhere I could see in the Terris or Skaa supplements, it IS explicitly addressed in the Alloy of Law supplement:
Unlike the Mistborn Adventure Game, Stunts are no longer the purview of just Allomancers and kandra — now, Heroes can also select Stunts that reflect their skill with a firearm; gunsmithing talents; innate koloss abilities; ways which they use their Feruchemical or Allomantic magic during gunfights; or Twinborn tricks.
Most Heroes can learn Stunts during Hero creation, depending on their choice of Powers, or can gain them in play by spending Advancements (so long as they meet the requirements). Note that the availability of Stunts for Heroes without Allomancy means that Heroes with weak Powers can choose one or two Stunts in place of Traits during Step 4 of Hero creation (see page 89).
• Your character has no special Powers but gains two additional Traits. You can replace one or both of these Traits with Stunts (generally speaking, only gunplay or gusmithing Stunts) if you wish.
SO the answer is yes, you can select stunts instead of your weak-power bonus traits during character creation. Everyone I know also extends this guideline to Networks, since they also cost 4 AP. Also note that Mistings and other average-power characters who begin with a stunt can select a skaa/terris/gun stunt or Network as their stunt.
How long do the narrative actions that come with tapping 10+ feruchemical charges last, especially for the ones like pewter or tin?
One beat, unless otherwise noted (such as Feruchemical Bonze or Gold). The only thing that changes for different beat lengths is the narrative description. For example, Feruchemist Frank is in combat (5 second beat lengths) and taps 30 charges of Pewter. Narratively his muscles grow to massive inhuman proportions for a few seconds and he proceeds to Hulk-smash his opponents. Later, Frank is helping to unload a bunch of heavy crates from a boat (beat length of an hour) and again taps 30 charges of Pewter to assist in the work. This time he grows to be a well-muscled (but not inhumanly so) man for the course of an hour.
I've seen a couple posts talking about Average/Weak/Mighty characters. Are those in Alloy of Law or is that a house rule?
Like Bill said, that was a house rule using the villain creation section in book 3 of the MAG as a guide. Personally if you want to start with stronger characters, I'm in favor of simply creating characters as normal and then giving them a bunch of AP to spend on advancements.