I have a wiki on the way that'll explain a lot of different strategies concerning slave training. I know it's a steep hill for a newcomer right now, but that'll improve in the near future.
Also, there's a new update coming out tomorrow (hopefully, or Friday) that changes the core game considerably by balancing, fixing bugs, making mercs more controllable (upgrade armor, pick them up in taverns instead of randomized, etc) and a load of other stuff that are too numerous to go into right now.
Next update (0.7.8) will see the slave cap increased to 4 after the player buys an apartment house close to his home in Redhaven. I'll jump this up to 6 eventually when I'm satisfied with the system and optimized engine load.