Two clowns in a 9 will have to be moved to a larger tank once they get bigger. You can keep a pair in a 20 gallon but a lot of places say 10 gallons per clown.
I wouldn't keep all of those in an 8 gallon but I'm sure it could be done. The fish may not be to happy with the living environment. My clowns swim all over the tank so having two clowns and a dotty back in an 8 gallon wouldn't give them much room and the two clowns may pick on the dottyback if they host a spot in the aquarium and he won't have anywhere to run.