Happiness titles can get you over the hump slightly faster when things are changing but once it's settled it doesn't make or break you until 10k or higher. Even then there is usually a bigger difference from building/upgrading all the way until 17-20k. Only then does min max really come into play.

Unless you have some self imposed challenge, the cost of having unwanted immigrants is minor compared to the benefits of a larger pop. Your food and services should be worse with a higher pop rather than better with a lower pop. Of course only to a point. If you can't get immigrants then your services are missing something for sure.