Clown gobies can under go bidirectional sex changes, so if you get 2 of the same species they'll breed. Breeding pairs can do lots of damage to sps colonies as the gobies tend to clear flesh off the coral to make a nest. Adding another species is a little hit or miss. I tried adding a black clown goby but he spent all of his time chasing my green and yellow clown gobies. If you can, try to introduce the gobies at the same time. I wouldn't pair a citron w/ a yellow clown as the citron is about 2x its size and could easily prevent the other goby from eating if it was so inclinded.