I actually wouldn't worry about the extra chairs/tables. My parents do a huge party every spring, ranging from 50-100 people and have been doing so for over 40 years. The menu is always the same- Lasagna and bread (salad, dessert, apps etc change, but always the lasagna)The lasagna is great actually, because they make 4 enormous trays of it (professional film developer sized) but can heat 1 or 2 at a time, which keeps people from mobbing the food, but also controls the amount well. People have never had an issue eating it off of plastic plates with plastic cutlery and will sit pretty much anywhere, or just stand and keep talking while they eat. There are seats for those who want, and the flow works fine, and it is entirely inside their not so large house.
If you are worried about flow, they do set up the keg/bar in a separate room from the food, and neither is in the kitchen. It seems to keep people moving to different parts of the house, and keep the crowd manageable.