For the Collision issue: assuming the rooms have a fixed area, you could assign each a value. For example: small room = 1, medium = 2 and large = 3. So if a wall can only hold lets say 4 places: you spawn 3+1 or 1+1+1+1 or 2+1+1, 2+2, etc… You can loop select random sizes until the wall has been “filled”.
Since they are spawning in a fixed area, you can use simple math to know what room fits where without the need to check after they spawn. Like in this quick test I just did. Here the bp is randomly spawning “rooms”, it’s taking their value and placing them accordingly until all the spots are either filled or loop is done.