You can recruit slave soldiers from your regular slaves after you research the slave soldier topic, yes.
And yes, picking slave soldiers locks you out of some tech, and makes it harder to get access to some tech (namely chateau de la mort and some others). I'm currently doing a run with slave soldiers and honestly, its not easy, at least until you get them some basic armor and access to better weapons, but you can't really can't really take advantage of their numbers until you get yourself a skyranger at the very least. I feel the game still needs some adjustments to make slave soldiers a worthy option.