Blender Interiors Workflow + Collision Advice

Hi everyone, I am working on my first game project and I have learnt the basics of Unity and Blender. I was wondering if someone could give me some advice. I have a house (1 floor) with multiple rooms which I have created in blender, I have imported it into unity as one model. My two questions are : 1. If I want to texture the walls and floors of each room separately do I need to import these as separate models and rebuild it piece by piece in unity ? 2. I have ensured that all normals are facing the correct way but I can't for the life of me figure out how to stop my character from walking through them. on a side note (if this is relevant), the mesh that has imported from Blender along with the model is much smaller and tilted 90 degrees . Is this anything to do with the issues I am having ? Thank you for your help.