They are adults - treat them like your friends
1) Girlfriend/boyfriend - say nothing bad unless you must
Rest easy - In the end, your influence over the 18 years will pan out.
2) Daily interaction - I request them to respect my schedule when they are at home.
They can't make noises if they come home late. I also ask them if they would join us for meals. If they said no, no food for them. Nothing left on the table. Do not ask them to follow your schedule.
3) Money - None.
Ask them to work for their own money. No pocket money.