My guess is they're basically addressing the 'Niko Bellic is an ok guy who occasionally goes on bloody rampages' dissonance thing by giving you one character who's more or less psychotic, and who can go on killing sprees without 'breaking character', and allowing the others to fill the more empathetic roles in the story.
CJ is the legit empathetic one, Tommy is the psychotic character you unlock achievements with and Claude is killed in the first ten minutes of the game (being CJ's impetus for getting out of the biz).
Venom, scaly patches and segmented, snake-like bones?
Snake men have a pod/snake-tail instead of feet, are full of eggs when autopsied, are slow moving, usually screen crysalids, resist damage from guns.
The thin men are described as modified reptillians yes, but they don't look or play anything like the snake men did. Snake men were tough slow moving hulks usually with heavier weapons. Thin men are agile and fragile, infiltration guys with light weapons. One thing is simply not like the other.
I realize they aren't literally snakemen, but it seems pretty clear they're a wink-and-nudge combination of snakemen and the MIBs from weirdo UFO mythology. I could have sworn Jake Solomon even admitted as much in an interview or live stream at some point, although I don't have anything concrete that backs that up so feel free to take it with a truckload of salt.
Don't get me wrong, I would love for a DLC map pack or whatever to reintroduce them in something closer to their original form, but I seriously doubt it's going to happen.