I think the fast and slow traits are best understood in terms of the maximum number of quick actions that the creature can take during its action phase. A slow creature can take one, a fast creature can take three, and all other creatures can take two at most. However, each creature can only take one non-move quick action as part of its action phase. So the fast creature that can only use one non-move quick action therefore has up to two move actions in addition to its non-move quick action.
A regular creature can take at most one non-move quick action, so it has 1 move action in addition to its non-move, unless it decides to use two move actions instead in such case it gets 0 non-moves.
And a slow creature has 0 move actions in addition to its non-move. It can choose to use its action to move instead of doing a non-move though. It can either do one or the other, not both.
So aside from the rules governing how many of a creature's quick actions can be moved and how many non-moves, the fast and slow traits basically boil down to quick action +1 and quick action -1. You could imagine a hypothetical "super fast" creature that could take three move actions and then a quick action. (Quick action +2). However because of the limited size of the arena this might break the game. Then again, I think there is a way to balance such a trait. Simply say "if this creature takes three move actions, then it can only use its remaining quick action for X." X could be a number of different things, depending on the creature.
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