Who to give my Fous to?

From my suggestion would be Shiki (one of the best 4* assasin ) or Eurayle (would help a lot in Camelot which is coming soon) .

Fou's are not THAT precious to hard thinking about them (4* fou's in far future will be but not now). Just give them to servant you using the most often.

Cu will never do you wrong. but you can get a servant's worth of 3* fous per month so just dump wherever really and focus on 1 in particular per month, they're not rare enough to really question who should get them

Shiki will never treat you badly.
Cu, either.
Hell, even Medea's a contender.

If you're concerned about the Camelot chapter, bump up Euryale. Othewise, most of that screen is fine to dedicate some cards to, Martha notwithstanding.