Build these close to houses which are occupied by the clans you want to mingle and slowly they will start to hang out and improve their relations.

Alternatively, you can segregate the clans on different islands suing districts and peirs and their relations won’t change.

How to Move People Between Districts

1. You need to unlock the pier in exploration tech tree.
2. Then build one in every district.
3. Then just select that pier and “transfer people”.