Palladians are recruited from hives. The vast majority end up in the PDF - they do a short training program, then go back to work in manufactories. They then get mobilised in the event of a major conflict, and are known as the Auxiliary Regiments. The Imperial Guard proper, ie what counts as part of the tithe, are the minority of volunteers who join the Legionary Regiments.
To put it bluntly, Palladians are all gangers. The hive-worlds themselves are not dissimilar to Necromunda. A bit less anarchic perhaps, but still lethal. Palladian 'training' is less about teaching skills than instilling discipline - the gangers already know how to fight, the Imperium just teaches them to point the guns in the right direction.
Training is brutally harsh. From the first day recruits are subject to full military law, most of which is centred around uniform regulations and drill. Minor infringements are met with extra duties (reporting to the Duty Officer on the hour, every hour, in full kit, for three days is a common punishment). Repeat offenders are flogged or beaten. For the more rebelious and insubordinate, repeat malingerers will find themselves in a penal battalion or hazardous mining unit. Answering back, striking an officer or running away is an immediate death penalty - usually on the spot, and often with the sergeant-instructor's bare hands.
Discipline in the Palladian Guard in one song, based on Rudyard Kipling's Poem Danny Deever
For those that survive the hellish six months, they emerge into a (comparitively) lax atmosphere. Their officers are generally canny enough to know how to treat their soldiers - violent gang fighters who have been through soul-bendingly harsh training, where mental imbalances are fairly common. Soldiers are kept at arms length with a great degree of freedom off-duty, of course leading to a few 'incidents', generally involving alcohol. But the Palladian Guard's attitude is, if no-one from the Imperial Guard was hurt then it's okay. Looting and pillaging, and worse, are effectively condoned.
On the Battlefield
The Palladian Guard fight in much the same way as they did in the hives. Infantry doctrine effectively revolves around manoeuvering platoons into position so that they can get into the viscious close-in firefights to which they are accustomed. The more intelligent and loyal troops find themselves in the Armoured or Artillery arms, and it is this fusing of an elite, intelligent professional minority supporting a rabid mass of bloodthirsty cuthroats which makes the Palladians so successfull in war.
That's all for now folks, hopefully a bit more of a realistic picture of life in the ranks. I'm on a bit of a rampage, toning down all the in-world style of my fluff to make it more accessible, so there may well be more of this to come. Until next time...