Why would fluid negate STSF at all? I mean, in theory it could based on what Fluid is meant to be, but I'm not sure that it does since STSF is all about observing the movements of the opponent, which would include the feints. If anything, STSF would negate fluid because they would observe the techniques used to bring about the feint.

It is an interesting question though, I'd be curious to know if it does anything or if the two legacy checks ignore one another.