You can hire a babysitter by using the call for services option on your phone. If all your adult sims leave the lot and the baby is still there the game will automatically ask if you want one called as well. They're not really all that good though. If you want a nanny you could ask a sim to move in so that they're there for baby care, but you can't hire one.
And no, you shouldn't need any expansion packs to have same gender sims get married. As long as you've got the box that says the other sim finds yours extremely irresistible and you're already at the boyfriend or girlfriend status, you should be able to propose marriage to any sim. I think the only thing same-gender couples can't do is have kids together, but I could be wrong.