Anemone crabs are totally harmless to anemones actually. They prefer anemones with short tentacules like Stichodactyla gigantea so they can stay on the anemone. Otherwise, they just hide underneat the anemone (when you have a bta or magnifica forexample) and usually either dont get enough food or get eaten by other creatures as they are very defenceless without an anemone. Some of them can live long in tanks without any aggressive tank mates though but its best to keep them with short tentaculed anemones.
There are many species of anemone shrimps. Most of them are in the genus Periclimenes. The most common one is Periclimenes brevicarpalis. This species feed on anemone tissue and i wouldnt recommend keeping these shrimps in home aquaria because they can cause a lot of stress for anemone which may lead to death. Without an anemone, they will starve. Another anemone shrimp species is Periclimenes holthuisi. Its much better choise than P. brevicarpalis and can be kept in aquariums. Periclimenes species are usually found on anemones with long tentacules like Heteractis magnifica. One of the most common anemone shrimp species is sexy shrimp (Thor amboinensis). They are also safe but are not usually as loyal to their anemones as Periclimenes species.
The behaviour of clownfish against crab and shrimps is related with the size of the anemone, species of the anemone, species of the clownfish and character of clownfish. Ocellaris usually wont mind the shrimps or crabs IF the anemone is big enough. They may even benefit from the cleaning sessions of anemone shrimps as Periclimenes species usually clean parasites of fish like cleaner shrimps.
