To OP's question:
most people do not experience success in keeping multiple individuals of the same species of tang in the size of tank that you have. Yellow tangs can be quite aggressive, and they may fight unless you are lucky and/or have many hiding places. You may have more success mixing different species of zebrasoma in your tank, but it's going to be hit-or-miss. For the record, I have a sailfin and a scopas tang in my 165g, and they've never fought, but it's still a gamble.