I have a fairly wealthy city where demand for low income residential is sky-high; all else is medium. Land value is very high, however, which is why I suspect low-density mansions are being built instead of higher density buildings. The other reason I suspect this is the case is that if expand into an undeveloped area (hence lower land value, at first), a few low-density sky-scrapers will go in but subsequent large plots will be mansions. Perhaps I am just trying to build too many sky scrapers, or is there actually a gentrification issue in my city?
One method I used to take care of this (I used to make the mistake of confusing low income with low density) was to create residential housing zones in the lesser desirable parts of town, like near the landfills, power plants, etc. The desire for this area will be low, but the lower income residents would still move there, where the higher income residents would stay in the more desirable parts of the city.
Side note, it's been a long time since I've done that, so I'm not sure if the mechanics are the same now or not. (if that's not the case any longer, I'll be glad to delete my answer.)
If you're trying to maximize population, I would actually suggest trying to get high wealth skyscrapers to deteriorate. You can do that by removing some of the services. And, of course, making the buildings you want historical.
Dilapidated Ong Condos, for example, hold more population than any other building in the game, per tile or per building :
20 778 people in 16 tiles . Thus the most populous city in the unmodded game is the city with the highest amount of buildings of this type.