Sims can't marry while they are YA's.
The game won't allow for more than 8 Sims per household without a mod - they do exist, however. The limit is there so that players with computers that are at or around the minimum requirement don't completely make their game unplayable by having too many Sims to keep that much track of.
Your post comes off with some contradictions to me.
You say you don't want crush their dreams but then in the same sentence say you want them out of your hair - like you don't plan on playing them past getting them married to fulfill their parents' life goals. If the latter is the case, why does it matter if they drop out or make 4.0's? If you do plan on playing them later, why the rush to get them through part of their life?
