Get races with useful combat stats, especially dodge, dex, or strength. High race multiplier helps too. I love Abyssins, some people love Hutts or Noghri, and Togorians are good too. If you hire them yourself you pay less, and even if you can't get access to the race homeworlds, you can hire a bunch and pick and choose the good races out of the lot.Edited By: Syn on Year 14 Day 359 16:06
The stats matter a lot more than the armour. Armour is a sufficiently small difference that you can just buy the NPC shop hide armour or something. Ideally you will want to avoid them getting hit at all, so armour should be inconsequential at this stage.
For weapons, ELGs are definitely great, but anything with at least a range of 4 is good for training purposes, so you might want to shop around and see what you can find at least 12 of. If you avoid bandits and focus on creatures, make sure you check the creature herd you are targeting has a weapon range less than 4, and stand out of their range to attack. Some creatures (like nerfs) have no attack at all so they're great to train NPCs on. Melee troopers can use ranged weapons too, so that's also an option for training them up. Just always remember to check creature range and stay out of it, while staying within your weapon range. Don't bother with bandits since they tend to have a broad range of weapons and checking them all manually is a lot more time consuming and boring than just moving on and locating a new creature herd.
(Sidenote on weapons: if your goal is to level your NPCs, don't have them dual wield. Use only a single weapon. The offhand weapon will always have a hit penalty, meaning it has a much higher chance of missing, meaning your NPCs take an XP reduction for their missed attacks. You can put proper gear on them later on. For leveling purposes, a single Relby is better than 2 Relbys, for example.)
Combat training like this is always significantly faster than patrolling your NPCs, but there's no reason not to have as many squads as possible patrolling at the same time as you take a squad out in combat to maximise XP gain across all your NPCs. You can also have friends set squads patrolling for you if they haven't already used up all their squad space. :p (Infantry Command skill is good for this and will increase the number of squads you can patrol)
Stay away from Derra. Other players can kill your NPCs there. Just hunt on some of the Darkness controlled planets in the galaxy, or on a planet your faction controls where you and your NPCs will be safe.