On most airlines, you can choose your seats in advance, and if booked far enough in advance can be guaranteed adjacent seats
No, you can't ever "guarantee" adjacent seats. You can reserve seats that are adjacent, and you can pay extra for a premium seat (window, aisle, or bulkhead) but in the end a seat assignment is not part of the contract and the airline has the right to move you wherever they want.
I don't think many airlines let you pay extra for adjacent seats. You can pay extra for a special seat (such as a window), but they can move you to another equivalent seat (a window in another row) without notice or compensation.