i am wondering if it is possible to edit orders (reducing the size) submitted to the order book? i trade multiple stocks at the same time and sometimes wanna adjust/reduce the order size of some stocks without losing priority on the book.
Read the exchange rules for the relevant exchanges. This applies to all exchange traded markets (including exchange traded stocks). The futures exchanges I trade on preserve priority when reducing size. Increasing the size (even back to the original size) will reset your PIQ, so better to add a new order to increase size, leaving the old order with the better priority. This applies to the futures exchanges I trade which are strict price-time priority matching for the instruments I trade. There are different ways to match orders; standard price time FIFO, pro rata allocations, etc. You need to check with the exchange in each case.