It focuses primarily on the XBOX because you won't bother with soldiers on the PC but use leaders instead. It does not contain story or quest spoilers but some game play info only available later in the game.
Why bother with soldiers?Edit
You are limited to six leaders in the active party on the XBOX. With a maximum of 18 active units, this leaves 12 free spots that can only be filled with soldiers.
When to hire soldiers?Edit
You should hire the soldiers when you have free spots in your active party. Only hire the soldiers you need to fill those. Hire them as early as possible to maximize the development of their stats and skills. Do not switch soldiers, hire and stick with them (see the development section below for why).
There are several factors to be considered when choosing your soldiers: their skills, their weapon, their max stats and their formation attributes.
Soldiers will not learn any additional mystic or item arts, so they are limited to the ones they know right from the start.
Unless you have a very specific union in mind, it is not recommended to hire soldiers with only combat arts. Choose the ones with mystic or item arts as they will also learn the combat arts for their weapons. If unsure take healing arts, they will save your butt often enough.
There are several factors to be considered for the weapons. All soldiers have a fixed weapon upgrade tree. They will only use those weapons and upgrade them on their own when they have reached the required Battle Rank.
The first factor of course are the weapon stats. Do not focus too much on attack here. The defense values play a big role in being able to survive some of the harder boss fights.
The second factor are the combat skills. The soldiers will only learn the arts for their equipped weapon so if you want specific skills choose the right weapon.
The third (and maybe most important) factor is the upgrade tree. You will find them on the page for each soldier. From time to time a weapon will be strengthened after a battle, giving it a stat bonus (+1, +2, +3, ...). The bonus gets bigger with each strength upgrade and also is bigger on the later weapons (i.e. a Draconile Harpoon + 3 gets ATK+20, DEF+7, MYS+15 and M.DEF+4 which makes quite a difference). These strength upgrades are quite rare so you should cherish them.
Unfortunately the game will ditch all upgrades when an "illegal" weapon upgrade is used. Let's take Deacon as an example. His weapon will upgrade from a Draconile Claymore to a Divine Claymore. There is no valid upgrade path between these two weapons, so he basically throws his old weapon away and creates a new one and doing so he loses all upgrades.
Focus on HP and AP for the max stats. Their other attributes like STR are mediocre, but for HP, you will find variances of ~1 000, which will make quite a difference. You will find a list with their max stats here (although it's mostly data collected from forums so it may not be 100% correct).
There are two ways to develop a solider: leave him at the recruiter or use him in the active party.
Unhired soldiers will develop over time. Although they don't improve their stats much, they will develop their skills. This is a good way to upgrade some of the more situational arts (i.e. a soldier starting with only Antivenin is almost impossible to level up, giving him back to the recruiter until he got Eye Cream makes further levelling much easier).
The second option is to use them in the active party. Soldiers on the reserve bank will not gain any stat or skill upgrades so the Rush only skill grind trick won't work for them! Skills will upgrade when used (like they do on Rush). Stat gains depend on the difference between your Battle Rank and the one of the monster you fight. Therefore try to not raise your BR unnecessarily until you hire (most of) the soldiers you want to use through the game. You should not switch soldiers later in the game as it is difficult to compensate the "lost" stats (the new solders will most likely have worse stats than the ones you used so far).
Soldiers will upgrade their weapons on their own. They will ask for items after fights, but unless you have a surplus of the item, it's recommended to keep it for yourself. All soldiers will "clone" the required items once you enter a town and have it in your inventory. They will also "clone" the accessories you have and automatically equip them. Both the weapon and the accessory upgrades are linked to specific a Battle Rank.