Generic male slaves undergo a transformation and are classified in three tiers based on their id number: punk tier, middle tier, and space marines tier.
Punk tier is for punks, young punks, old punks, and anyone with an id of 0. They typically have shit starting discipline (their processing display screen says -15), prone to Addiction, and shit intelligence.
Space marine tiers are at id 7 to id 15. They are not all space marines. The warlord of the Hide Your Daughters crew for example is id 7. They can have huge starting health and their processing display says +15 discipline.
I keep every workaholic male slaves I get as well as some perfectionists with good starting discipline. Enough to fill one workshop. And then I sell the rest immediately in batches. I process them in batches too since if you do that, the time does not progress from the processing/selling of the first one. I sell them to afford building the workshop and afterwards I just send every capture I get to Fort Sera. I need the fort to get 200+ men for the Fort Sera political event that raises Republic Standing to proc. It might not do much now but its a good way to raise the notoriously grindy republic standing. Also, increasing Fort Sera's power increases its income slightly. Its not much but I would rather gain the small income increase from him than sell him in Watery Eyes for a quick $4k when I am 100 days in. At day 10, I would have sold him. I am probably just too lazy though.