Hi, Eero.
The method you propose for the short creation is not a fixed rule, right?
So can I create a short NPC like this?
NPC 1
Pool: Vigor 3, Reason 3, Istinct 5
Detective (2)
2-3 Secrets
Thank you.
From my perspective, I think that is fine. For NPCs that come into the game unplanned, I tend to add stats as I go. If there is no conflict with the detective NPC, then he doesn't need any stats at all. If it's just an opposed check, he only needs Expert (2) Detective (I) and perhaps a bonus die from Pool expenditure, unless you are wanting to showcase a really cool Secret as well. If the detective NPC is a major player in an extended conflict, then he also needs an appropriate Pool, a Harm Track and probably a couple fleshed out Secrets, as you list above.
Note that I wouldn't bother fully stating out the detective if he was only a bit player in an extended conflict, such as helping out one side or another, but not actually being the target or source of the conflict. Just give him an appropriate Ability and the associated Pool and run with it.
Yes, you're both correct insofar as my play practice goes: NPCs can be created any which way, the important part is that the GM has discipline and can handle the amount of bookkeeping he chooses to use case-by-case. When a campaign is in a situation that requires and allows an important NPC to be created in advance I well might give them more detailed statistics; this is not very often, as most of the time the players determine which NPCs grow to be important and which just have bit roles in the campaign.