@ShivuKaan Hello there and welcome to the forum!
I didn't understand the question about building in your first paragraph.
As for your second set of questions, I'll try to answer some of them.
*Premade Sims: Yes, you completely remove them by going to Manage Households and deleting the household.
If you want to keep them in the world but not in a house, simply click on the lot and choose "Evict" to kick them out of the house.
*What do you mean by "live and act normally"? Unlike Sims 3, Sims 4 has no story progression.
Do you want your various households to get married, have children, and so forth. You'll have to play a household for things like that to happen.
The only autonomous activities will be forming friendships/romances and getting jobs. Sims cannot get engaged, married, or pregnant without you.
If you want to play one or two households with your other custom sims for them to interact with, then you're fine.
Best wishes and happy simming.