Initial meld and additional pair of same rank
Yes, you can do this, but you have the order wrong. You don’t first meld, and then pick up the discard pile. You need you first show 2 8s from your hand, and lay them down along with the enough other cards so that the total meld is at least 120, which can incude the value of the 8 from the discard pile as well. 

From these rules:


  Therefore, in order to achieve the minimum count, you must either meld entirely from your hand after drawing from the stock, or you must use two natural cards from your hand which match the top card of the discard pile. In this second case, you can count the value of the top discard, along with the cards you play from your hand in this and any other melds, towards the minimum count. You cannot count any other cards in the pile which you may intend to add in the same turn.