Yes, you can choose the gender if you adopt a child. No, it's not allowed in the Immortal Dynasty Challenge. All children must result from natural pregnancies.
One way you can find new people is to read the newspaper every day. It will tell you the names of new families that have moved to town. If your Sim is already a young adult, you can get to know the NPC Sims in town, like maids, repair people, firewoman, etc. The park during the day is also a good place to meet townies. Most of the NPC Sims visit the park regularly. If your Sim has level 5 in any musical instrument, he can play for tips in the park or at the theater. Anyone who comes to watch becomes an acquaintance and eventually a friend. Then they show up in your relationship panel and you can see who is the right age. Also, if you throw a party, you can see the age of the Sims by hovering over their names.
I hope this helps.