Apart from the normal unit size cap I don't see anything in the book that would prevent me from taking a 50 strong adetachment to a 20 strong parent unit. Am I missing something?
Light Infantry (Handgunners/Crossbowmen) have the "support unit" special rule regardless of the units size, but they are limited to 20 models anyway.
Heavy Infantry (Swordmen, Spearmen, Halberdiers) have the "support unit special rule (20 or less models)" and have the "parent unit special rule (21 or more models)".
State Millitia (Free Company/Archers) have the "support unit special rule (20 or less models)", and loose this if you buy a bigger unit. They don't have the parent unit special rule.
Imperial Guard (Greatswords) have the "parent unit" special rule, regardless of the units size.
It's all clearly laid out in the units special rules.
You can't take a 50 strong unit of Light Infantry as a "detatchment" because they are limited to 10-20 models in a unit.
You can't take a 50 strong unit of Heavy Infantry as a "detatchment", because they loose the support unit-rule and gain the parent unit-rule if the unit is larger than 20 models.
You can't take a 50 storng unit of State Millitia as as "detatchment", because they loose the support unit-rule if the unit is larger than 20 models.
You can however, have 20 Crossbows supporting 21 Heavy Infantry if you would like that.
Also worth mentioning, is that you don't purchase support-units (detatchment) to specific parent-units anymore.
A support-unit within 6" of a parent-unit simply makes supporting actions now.
What this means, is that if you have a support-unit of 20 Crossbowmen, within 6" of a parent unit of Spearmen, and within 6" of a unit of Greatswords, they can choose to stand and shoot (or counter charge) if something charges the spearmen, and if something charges the gratswords in the
next turn (assuming the crossbows and greatswords are still within 6" of eachother), the crossbowmen are free to stand and shoot or countercharge whatever's charging the Greatswords as well.
Peviously, said unit of Crossbowmen would've only been able to help either the Spearmen or the Greatswords (depending on what you unit you bought them with.)
Support-units also count towards the core minimum (but perhaps it also did in 8th Ed? I can't remember.)
So if youre bat crap crazy and field 5x20 Crossbowmen, and manage to keep at least one model from each unit within 6" of a parent unit, all 100 crossbows get to fire if the parent unit is charged!