There are accounts of people keeping fischeri together, but I would never assume you can always do it! Chams are individuals, cages are often not large enough or densely planted enough, and if the setup in the cage is not perfect it won't work no matter how many or few animals you have. I've had fischeri who were tolerant and those who were not. So, my answer would be this: don't expect to keep them together in anything smaller than a cage at least 5'x5'x5', but if you find out your particular animals will tolerate it you are lucky. Remember, even if you do house tolerant animals together you will at some point need to separate a gravid female, a very macho male during breeding season, any animal who is not up to par for some reason.