I think I posted this in the wrong section last time, so I am reposting it here...
Hey guys,
I need help figuring out how to best use detachments tactic wise.
The first Empire 8th ed game I played, I used a horde of 40 halberdiers with a war priest and a BSB, supported by two detachments of 10 halberdiers, one on each side. Now, my opponent (VC), charged my first detachment with some cav. and before I had the time to reply with my parent unit and/or supporting units (knights), the detachment lost the fight, broke, got run over and made my knights flee of the table due to their total destruction. Yes, I had the BSB, the steadfast rule, the hatred, etc., but you can still miss a leadership test with all that...
What I'm trying to figure out is how to avoid my opponent jumping on those more vulnerable units first? Especially if I were to use shooters. Should I employ less detachments but with more guys, should I deploy them a bit behind my parent regiment, should I remember the steadfast rule even if I loose 5 guys in one go? Did I answer my own question?
Anyways, I'm open to your ideas...
Corax 
Just so you know, I interpret the rule as you do with the steadfast being passed onto the detachment based on the parent's ranks even if unengaged. My opponents have no problem with that, but then again I only play friendly games.
I'd say if you failed a leadership test with a steadfast detachment using the Hold the Line! special rule from the BSB plus the reroll then it was mostly a fluke. I think this was a great way to use the detachments and if it weren't for unhappy chance you would've had a sweet flank charge on your turn with a big fat regimental horde.
And possibly, assuming that it wouldn't put them in danger of getting flanked, you could reform the regimental unit to face the enemy unit (and also reposition your detachment) so that you can follow up with a charge and/or countercharge. And, assuming you have the shooting support, this can possibly give you another round to fire on an unengaged unit.
Really these smaller detachments aren't that vulnerable. If your opponent continues to charge them he's going to more often than not end up in a bad spot and would've been better off charging the regimental unit.
Also, don't be shy about charging with a regimental unit and a detachment at the same time. I did this in a recent game and broke a unit of 40 night goblin spearmen with shields and nets with a depleted unit of 13 halberdiers and a detachment of 10 free company thanks to all the extra attacks I got and the fact that he could only net one of my units. Yeah, not as good as a free flank charge, but hey it can surprise your opponent.
Smaller detachments have the benefit of giving you points to put into your regimental unit, but they also increase the chance that your opponent will just blow right through them, and you give up the opportunity for disrupting enemy ranks.
I would keep doing it like that for now, and see how it works out for you over the course of several games. If you still aren't sure of the answer then post again. I don't think you should base your opinions of the new rules based on one isolated incident.