i think the issue is more that they generally won't have the larger bodied fish to clean in a smaller tank. i mistakenly was talked into one by an lfs just like yours tried, it was eating mysis so i thought all was good. they also discounted the fact i had a sixline wrasse already in my tank. one day the sixline went after the cleaner, cleaner disappeared for several days, and then one day i saw my cleaner shrimp carrying the corpse of the cleaner wrasse across the tank for eating. not once did i ever observe my smallerish fish allowing the cleaner wrasse to clean them. other lfs i've talked to about it have said they just don't bother with cleaner wrasses because they are so hit or miss even in large tanks.
i'd go with the neon goby if you really want a cleaner fish, although i'm not sure they serve much purpose. i have a cleaner shrimp for aesthetics, not because my fish let him clean them. they aren't going to cure ich in your tank, which was the main reason for the lfs telling me to get one.