Population Limits

PQ limits the population, but nothing prevents you from downloading a plethora of colony ships on a single world. They'll probably have a morale of 1 anyway.
100 billion doesn't look so huge. If the whole surface of the planet can be used (like it's arid as Mars or you can settle oceans), it gives a lot of room. Currently, half of Earth population lies in 2 countries (China and India). If the Americas were as densely populated, you'd have a population of more than 10 billion. Then settle Africa, get rid of the deserts in Arabia, central China, America and settle these. Settle teh oceans too, while you're at it.
Comparing the size of India and China with the surface of Earth makes 100billion look conservative. Our Earth is PQ 15. A PQ 20 planet would just have to have a diameter of 1.1 times the Earth to get 33+% more surface for instance, and gravity wouldn't be increased that much, depending on the mass of the core.

I had no idea that earth could support so many like that. I thought that we would just build massive high rises for living space.

First we have to kill off all the wildlife before we do any heavy construction. Reversing desertification is slightly more difficult. Recently, the Chinese are making headway in the Gobi Desert to stop the loss of land.

For actual living space, you could go underground and house people in subterranean caverns--you get almost unlimited space that way. And that would leave the ecosystem mostly intact.

Feeding the population is no problem if you have Artificial Life and Replicators and things.