Hey there! Fast, as a modifier, makes it where you have to choose between moving the extra six inches or making an attack in the same turn. Acrobatic allows Fast units to make an attack where they wouldn't normally be able to. Nimble allows a unit to attack and move outside of the normal move / attack cycle. If you want a fast unit that can also make a ranged attack, you'll need Acrobatic. Nimble just offers more flexibility.
Terrifying can be used to affect a friendly unit, but since it counts as an attack (and the unit isn't required to attack) then it doesn't have to affect any friendly units. It can, though, if you need that flexibility for tactical purposes.
You can totally use those as 1-point modifiers! If you're worried about granular balance issues, you can always do a trade-off where your opponent also gets an equal number of modifiers from the Arcane Training document.
Cheers!
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Thank you for getting back to me so quickly, Ellie! I was wondering about how Ranged and Acrobatic interact, in particular, because Ranged says that "the unit may only attack at range if it has not moved in the same turn." My thought was that Acrobatic might override that rule, since it lets the unit "make attacks no matter how far it moves." Is that what you intended?