How would using only one s-box affect security of Blowfish?
Because I didn't get any answer, I did some experments with Blowfish's F function and here are my observations:

Because Blowfish's F function is not bijective, on average only every second value is returned and therefore different input can output same value (F(x) = F(y)). By reducing number of S-boxes this non-bijectivity becomes even worse. About 8 to 32 of inputs will result in F(x) = 0. But problem is worse, because addition hides problem; With F() with XORs instead of additions about 2^18 inputs will output 0. This happens, because (S(X) xor S(X) xor S(X) xor S(X)) or (S(X) xor S(X) xor S(Y) xor S(Y)) ... is always 0. So addition breaks symmetries a bit.

I have also found comment by David Wagner on question about strength of Blowfish with broken F (((S_1[a] + S_1[a]) ^ S_1[a]) + S_1[a]). He overlooked that also the same input byte is used. So he was answering my question.


  For what it's worth (which is not very much), I don't immediately see
  any plausible way to exploit this on the full Blowfish, though I
  haven't spent any serious amount of time thinking about this.  16
  rounds, with key-dependent S-boxes, forgives many sins.


https://groups.google.com/forum/#!topic/sci.crypt/21SwIdSqOo8

Considering the S-box is unknown, addition and 16 rounds I think it should be fine, but I don't think it's a good idea.