Slaves that have their HP based on S.P.E.C.I.A.L. stats provided in the wiki can level up.Just checked. I have captured ghoul Scavenger with 57 HP near Vault City, after levelup he got 62 HP. ... Time to get some 53 HP crazies and create crippler squad.
No luck in leveling up 69HP combat slaves.
You should capture 60 HP combat slaves.69 HP combat slaves do not level up.