I must be missing something in the book that talks about this. From what I'm seeing you can cast slow on someone who is quickened which would stack since they are separate conditions that don't mention interacting with the other.
Normal: 3 Actions
Quick: 3 Actions + 1 Stride/Strike
Slow: 2 Actions
Quick+Slow: 2 Actions + 1 Stride/Strike
If anyone can direct me to the relevant text that points to the interaction between these 2 conditions that would be great otherwise I guess this is how it works.
Page 324 in the Playing the Game chapter under Conditions says:
You gain 1 additional action at the start of your turn
each round. Many effects that make you quick specify the
types of actions you can use with this additional action. If
you become quick from multiple sources, you can use the
extra action granted to use any single action granted by
any of the effects that made you quick.
You can spend fewer actions. Slowed always includes a
value. When you regain your actions at the start of your
turn, reduce that number of actions by your slowed value.
You can’t Ready an action when you’re slowed. If you
become slowed during your turn, you don’t lose any
actions until the start of your next turn.
The quick condition definitely puts the restrictions, such as only a Stride or Strike action, on the extra action, not on the character. Slowed talks about number of actions and about spending fewer actions. If a character has both the quick and slowed condition, then at the beginning of the turn, the player could first gain the extra action from quick, and then second lose that particular extra action, the one with restrictions, to slowed. When slowed removes the action with the restrictions, the restrictions disappear, too.
However, that character won't be able to Ready an action while both quick and slowed.