ALL PCs with Recovery or Toughness need to attach a Catch of some kind at my table. You don't "get" to take one, you must take one, because there's a silver bullet for literally everything supernatural in the Dresdenverse, it's just a matter of figuring it out and getting a hold of it.
I might say that some catches are so rare that they might as well not have one. But, in cases like that, they don't get a rebate anyways.
As far as Human Form and Catch rebates...they're just completely separate things. If both are appropriate, you get both. Human Form is for when you *don't* have some powers 'normally' and *do* get them in some shapeshifted or conditional situation. Whatever your Catch is, the rebate it provides still can't reduce the total costs of Toughness/Recovery below -1, and ALL the powers attached to Human Form must add to overall cost -1. Here are some examples:
"Not Allowed" Example - The overall Toughness/Recovery/Catch cost is -0. You're not locked out of using Fire, but you would need modulate the bonus down to +3 to make the math legal again for a total -1 cost.
-2 Inhuman Recovery
-2 Inhuman Toughness
+4 The Catch: Fire
This isn't exactly how I interpret how the rebate works. It's not that you *can't* have a +4 catch, it's more that the catch can only provide you with a +3 rebate - even if it's valued at +4 - because you don't have enough refresh spent on toughness/recovery.
So, I'd allow this set-up, but your total powers under the catch would still add up to -1, despite the +4. If the player upgrades to Supernatural Toughness, the +4 bonus would come into play.
"Not Allowed" Example - This isn't allowed because your overall cost here is -0, so you'd have to modulate the cost down to -1 overall on your sheet or buy more powers to stick under Human Form.
+1 Human Form, covering:
-2 Inhuman Recovery
-2 Inhuman Toughness
+3 The Catch: Cold Iron
Exactly right.
I have an idea for a character based on Moon Knight who is the champion of Khonshu, an Egyptian moon god. He would have Inhuman Strength, Speed, Toughness and Recovery as well as Cloak of Shadows, but only at night. I suspect that assigning these powers under Human Form would be appropriate to get a 1 point Refresh reduction on the abilities, but could/should I also say that the circumstances are "Rare" to reflect the fact that these powers are only available at night (and get a further 1 point Refresh reduction in addition to the one above).
Can I also let the character take the Catch (only at night) on his Toughness/Recovery Powers that can be discovered/deduced with access to rare texts once the sponsor is known for an additional reduction on top of the Human Form bonus?
I'd give someone who can only use powers at night, or when the moon is up a +2 ' human form: involuntary change' rebate.
You'd need at least -3 worth of powers there.
If you took toughness/recovery, I could see you having things like 'light' or sunstone as a catch. Maybe even 'moonstone'.
Khonsu is the god of travel, protection and healing and "light in the Night". I could see a catch of necromancy and/or entropy being a catch. Since travel is in his portfolio, perhaps being bound/helpless would be a catch. So while movement is blocked against him, he loses his toughness. In fact, his toughness could be skinned that he's so fast, he dodges and is hard to pin with a solid hit.
Time magic might also affect him since he's related to the passage of time.