in Hoops Dynasty a Freshman would increase work ethic a lot if he started. Any faster increase in stats for a young guy playing in the Majors?

I have only played one season recently but when i started the rule was to not get your top prospects in over their heads. If a player is good enough for the bigs put him there but if he is overmatched it could hurt his developement.

That's not a rule. And it's also not backed up by more than conjecture and one line in the sometimes misleading Help section. I've put overmatched prospects in AA and AAA and if there's harm to their development, it's not evident. Some believe it to be the case and others do not. If someone told you that's a certainty, be skeptical of their advice.

If there is a detrimental effect from being in over their heads, it has to be weighed against the benefit of (typically) better coaching at higher levels.

I'd imagine the closest thing to work ethic in HBD is Makeup, and we have nothing to indicate that is driven by role.