Can I send a teen to Uni but still play the base family?

Yup. As others have said, The Sims 2 is really built on rotational play. So sims outside the household won't age, won't progress in pregnancy, won't graduate, won't gain skills, won't get promotions or demotions, won't change jobs etc. These only happen to sims you're currently playing with. This does cause things like townie children your sim had brought home befriending their children, still being a child.

So you can switch there and back and other household's life won't change.