I've been trying to figure out what to do with my city. Is it really necessary to place every summoning? I have visited my fellows in the FS and some of the cities are huge and difficult to navigate. I would like to spread things out and be more visual but at what cost? One of my neighbors has cleared the city of almost everything but the roads and the essentials. I am going to keep watching and see where it ends up.
If you place everything, you will indeed have a very difficult time managing your city.
If you place everything you win, you won't have enough space set aside for guest race settlements, making those chapters take FAR longer than they should. Also, some event buildings are simply more efficient than others. For that reason, it is good to occasionally compare what you win to something else in your city. If it's better and space is an issue, then sell the old thing and replace it. I see way too many advanced cities holding onto things that are from a very early chapter. Unless you truly love the design of something, outdated buildings should go.
It's also good practice to always keep a bunch of things in your inventory. Some quests ask you to get spell fragments and having things ready to break is helpful. Or in a pinch, you may discover you need a little more population to upgrade something and having a building you can pull out to help that is nice.