None of it matters because "focus" is both a noun and a verb in the english language.
No matter how you try to make it plural, it will be wrong.
Edit: The main thing to remember is that the english language is convoluted and either Focuses or Foci are acceptable.
Edit Edit: Get outta here with your latin bullcrap. This is 'murrica.
2011 SES 5spd #006 >> SR #213
What other people think of me is none of my business.