I played the starcraft campaign and watched other media to know why they use criminals as soldiers. It all came to me after rewatching the original Full metal alchemist. In full metal alchemist, criminals are experimented on to make them into weapons for warfare or research whatever the fuhrer wants his soldiers to research. Starcraft is saying the same thing about their world.
Also I believe it’s based on history also. In Europe they use to make their soldiers be criminals first or they would take a criminal and make them a soldier. I have no idea why they did this.
In the fictional solar system the kuprulu sector soldiers are experimented on to make them better soldiers and better weapons for the war. Take Ghosts in starcraft which are snipers with lighter rifles so they don’t have to lie on the ground. Ghosts are kept secluded long enough until they become psionic. Firebats are probably trained on volcanic planets. I have no idea how medics are trained. Also some of these soldiers have been rehabilitated like Kerrigan. Kerrigan use to be pale with red hair. But she was sacrificed and infested to take over a planet.