There is a recommended limit of 6 apartments having more can lead to issues with severe lag. Unfortunately NPC sims do not interact much with each other. Having a grill in the common area means a lot of free food for your own family if they live there. You will also note that in parks and elsewhere it is popular to grill. You can see more of the tenants in common areas with training equipment's and computers there. They might even chat with each other while using these. Making 1 room apartments will have zero influence on how much they interact.