To start off with the basics, every soldier shares the "pistol" column which consists of the 7 pistol perks: faceoff (shoot pistol at everyone), fan fire (fire pistol 3 x at one target), lightning hands (free action shot), clutch shot (perfect accuracy pistol shot), return fire (one free shot at someone who attacks you), gunslinger (fire a reaction pistol shot within 8 yards), quickdraw (shooting with your pistol first does not end the turn). The order in which you can get them is entirely random (sometimes you can train faceoff immmediately).
The other two columns are "defensive" and "offensive." These columns are only 3 rows deep opposed to pistol's 7 moves. These abilities are randomly chosen from a pool outside of your class. So you might get the HEAT warheads perk as a specialist, or cool under fire as an assault.
Abilities take some time to train, not sure what it is on legend but I think it's maybe 5/10 days; 5 for "lesser" abilities and 10 for "better abilities".
You can eventually train all pistol and defensive/offensive options provided you are high enough rank. I believe it takes tech sergeant to be able to learn everything. You are also offered 3 abilities to learn from at a time, so if you don't like what you see you have to train something to unlock further tones.
Interestingly, you can only see the first 3 perks off the bat (1 pistol, 1 defensive, 1 offensive), and even if you check the "view hidden perks" option you can't see what will be your tier 2/3 defensive/offensive. So there is a bit of a risk of not having great options in the higher tiers, although the higher tiers are generally "better" than the lower tiers.
With this given information, here are my thoughts on who to train / how to train
1) The Greedy Strategy - simply examine each of your soldiers and take a look at the 3 first options. Sometimes it's immediately obvious that something there is awesome - examples would be the "flashbanger" perk (+1 free flashbang) and the +1 mobility perk. These are quick to train and excellent for virtually all situations. So just go ahead and train it.
2) The Pistol Strategy - since you are guaranteed to learn all the 7 pistol skills if you spend the time, you might carefully select a soldier that you would want to equip a pistol on and who would take advantage of those skills. A sniper makes sense because it compliments the rifle's poor short range capabilities, and snipers typically have high aim, and they typically carry ammo, maximizing the use of the pistol. A ranger might also make sense because they have good overwatching abilities and also generally carry ammo.
3) The Favoritism Strategy - there is just someone special in your squad and you just want to load him up with all the possible best abilities ever
4) Fishing for Potential Strategy - you're aiming to a late game hero squad and you have some specific ideas in mind. Maybe you want your gunner to have Damn Good Ground, so you might dig through all the offensive slots quickly in order to try to see if you can get it.
5) Lightning Hands for All Strategy - maybe you like lightning hands so much that you want to give it everyone, so you train just enough to get to lightning hands for everyone slowly rotating them out. Who doesn't like free action damage anyhow? Just remember you do have to equip a pistol to use it.
What I haven't really figured out how is how soon you should buy the second training slot (I think just delay it, too many other things to spend supply on that are guaranteed), or if there are other strategies I'm missing here. Thoughts?
