your tank is honestly to small for silver dollars, for they like to be kept in schools, and that would severly OS your tank because they get to be around 6 inches long. IMO you shoudl take them back before you get any other fish. that will leave plenty of room for new fish. since your angels is already very big, it is not advisable to add a very small one to your tank. it will likely get harassed by the big angel and die. if you can find angel that are the same size, you could possibly have a pair or trio, but watch out for aggression.
if you cannot find angels that are the same size as your current one, a school of 12 red eye tetras and a school of 5 corydoras and the current angel as the centerpeice will make a great scene.