What words are allowable depends on which dictionary one uses. Players should always use a good reference when they play. In most cases, yes, 'ef' and 'kay' are allowable. It's important to understand that we are not talking about foreign words - that's a separate issue. We're referring instead to the English spelling of either Greek or English (or Hebrew, etc.) letters. In each case, they are English words.

So 'phi' and 'aleph', 'zeta' and 'beth', as well as 'ef' and 'kay' are all permissible (usually).