Summary of the guidelines from YS327:
Nameless NPCs (thugs, mooks, and the like) generally don't get consequences, and also get little more than a couple of skills and a couple of stress boxes. Supernatural creatures that fill a Nameless NPC role get whatever's on their template, but no more. Generally these should be the types of creatures that players can plow through in (small) hordes.
Supporting NPCs get a bit more detail (more skills and perhaps one or several aspects). They generally don't fight past a moderate consequence.
Main NPCs get everything that PCs would get.
That said, it's really up to the GM what opponents are put into each category, and you can even add in extra gradiations if it seems appropriate (for example, slightly more resilient thugs that get a minor consequence and an aspect). There should probably be only a small number of Main NPCs in a given scenario. Maybe the primary bad guy and a lieutenant or two. If there are multiple factions involved, each one might deserve a main NPC or two. Any time you want to just throw a handful of popcorn foes at the players, that's a job for nameless NPCs, who will generally rarely take more than one semi-decent hit to take down (and can be taken down by the group with AoE attacks). Supporting NPCs would be good for adding a little backbone to a group of nameless grunts.
As to rituals and complexities, the caster can set the complexity anywhere he sees appropriate ... but you should not tell him how many consequences the target has, nor how many stress boxes, nor what defensive skills he might have. The safest bet, then, would be to assume the worst, but the caster might not have enough resources to set up a ritual to turn a Mythic Toughness character with Endurance 5 (dual milds) etc into a frog. Assessments might be useful here. If the caster does some research and finds out the the target is just some two-bit thug that Marcone hired yesterday from Thugs R Us ... well, maybe the ritual can be scaled down a bit without sacrificing effectiveness.