Unfortunately, I agree that the clown probably has a better chance than the anemone in an 8 gallon tank but IME with hosting clowns, they do tend to be much more 'stationary'...which I only point out as an attempt to understand why your LFS wouldn't have advised you differently (other than trying to turn you into a repeat customer anyways).
IMO, the problem lies heavily in the fact that anemones, like many inverts, are osmoconformers....which means they live (and die) by maintaining the same salinity inside of their bodies as the water which surrounds them and this may also apply to other items as well and if I understand things right (which I sometimes don't, of course), this may be one reason why some inverts are very 'sensitive'. Anyways, I think your first challenge will be with evaporation because, as the water leaves, the salt and other chemicals are left behind. For example, if 1/2 a gallon of water evaporates out of a 100 gallon tank, there is no problem, but if a 1/2 a gallon of water evaporates out of an 8 gallon tank, big changes occur. Granted, I understand that many people have 'nano' tanks but in reality, I'm sure that the wisest stocking options are very limited and do not include a very wide range of osmoconformers unless some significant safeguards are already in place.
Again, I may be completely wrong but if nothing else, this is one of those good examples of why and how trying to base livestock options on the number of gallons of water a tank can hold are flawed...and why I think you may be able to gain quite a few "sure!" replies to your 'clown fish in an 8 gallon?' question if you ask enough people; although I'm definitely on board with fijiwigi.
-TG
