That is generally true but not a conclusive answer. Under the right conditions you can keep more than one species of clown in a tank.
1) The tank has to be big enough
2) They have to be introduce at the same time
3) They need to be captive bred (in most cases)
4) There cannot be 2 females of the same type
5)The female cannot lay eggs or she will (certain species) kill every other fish in the tank
I am planning on having 7 ocellerous, 2 cinnimons and 2 white striped maroons (all captive bred and introduced simultainiously) in a 90 gallon tank with 3 Ritteri anemones. I will not sit here and tell you that this will definitly work, careful observations will be kept, but it is possible.
I think it depends on the situation - I have a 135-gallon with a huge maroon clown and two true perculas. They all get along fine. They were not introduced at the same time but the tank is quite large. I have seen up to a dozen perculas kept in a similar sized tank with no issues.
Just my experience. I had a tomato clown that fought with every thing in the tank! He took a trip back to the LFS (smile).
You do have a better chance keeping multiple clowns of the same breed but introducing multiple breeds into a tank is usually gonna end badly... it gets even worse if there are not enough anenomes for everyone... be ready for anenome wars. I have a 300 gal tank now and i would not put that many clowns together in that tank because it is too small for that conflagaration.
I had a mated pair of ocellerous and moved them from one tank to another at the same time I added 5 more(tank bred), they don't fight at all. But they are also the least aggressive of the clowns. They will share an anemone no problems (all). The shedd aquarium has I think some where around 75 ocellerous sharing 1 giant anemone. They are not the problem. The problem I am seeing is if the maroon (very mean but not so mean if they are tank bred) and cinnimon (almost as mean) should happen to quarrel. I am sure there will be at least a small amount of bickering. Clowns are easy to catch so its not like I can't move them if it becomes a problem.
I have in my 125 a pair of breeding occeleris clowns, that lay eggs on a regular basis, a black saddle back, and a Clarkii clown. They all get along and have been togather for quite sometime. I do however keep my mated pair of Maroon Gold bars on my 92.