a royal gramma or maybe an orchid dottyback would probably fit in nicely. feed your tank well and move your rock around if you dont have a lot of coral. that helps to break up the teritory when you introduce new fish. if you decide to get all your new fish at once you could catch your yellowtail, put him in a bucket, add your new fish then add the yellowtail back the next day. also once your clowns pair up they might take over the whole tank. they are a damsel and can get fairly agressive . less or more depending on type. i have 3 yellowtail in my tank and they dont usually bother anyone. they are probably 4 yrs old now. you are going to have to be very careful about adding new fish in your tank. its not that big and its easy for a fish to regard the whole tank as his. try an aclimation box for new fish. 2 gallon jar with mesh for a lid works great and its cheap. try to get all your fish in there fairly soon and at a young imature age. as they grow up their normal personality and agression will come out and new fish will get killed. i would leave any new fish in aclimation box for at least a couple days. and yes its stressful but as i told the last person i'd rather sit in a box while the bully gets used to me then have him punch me in the face every time he walks by.