there is no garantee, they often times won't pair just with an anemone, my clowns live in my hammercoral, it really is totally random as to whether they take to an anemone or another kind of stinging coral. Depending on the type of anemone they can be dangerous to other fish, some have even been know to eat their host clownfish... my blue carpet ate a goby, and a dwarf angel, but he was a very large carpet, they tend to be the more agressive of the anemones available in the trade
hope this helps
Brahm