yes, if your data structure is correct it should only fulfill the line items you specify in the array of line items to fulifll. If the array is empty it will fulfill all items in the order.
If this is a test system you can cancel the fulfillment (which will create a new fulfillment order) which you can fulfill again and again until you get the structure and information correct. Be advised that the id in filfillment_order_line_items is the fulfillment_order.line_items.id and not the order.line_items.id
Cheers
Gary