I would not suggest doing this. A 40 gallon is far too small to mix any clown species, it will only result in the death of your fish. It is also , IMHO too small for a sebea in the long run. these guys will get fairly large and need a minimum 55 when they reach their max size. get the pair of percs you will be much happier with them and given the dimensions of your tank will make better community fish.