I don't think you need a barbed nipple at all.. although that seems like the conventional thing to do. The few time's I've made a batch big enough to use that pot, I put 5/16 vinyl tubing on it and, believe me, unless you are transferring hot wort, it won't come off. I get that tubing about half way up the barb. I'd expect you could use 3/8 and get it to go to the top of the taper. When I used it, the nipple was dry and the tubing was dry. It takes a bit to get it up the taper... so, it's a tight fit. The nipple is threaded and has an 0-ring seal. Don't know what the thread is. likely metric as it's from Italy. I bought this from More Beer a number of years ago when I made larger batches.. don't do that any more
BTW thanks for the reply.