I disagree. Just because humanity came into contact with the violent, carnivorous Kzinti early on doesn't mean they would automatically assume all or even most aliens were dangerous. Also, "boomers" came into contact with several hostile alien species and surely some reports reached the general population of Earth so I don't see the problem you do. Not only that, Enterprise had weapons and an armory officer so humanity wasn't totally na´ve from the time they launched (despite not having the phase cannons online because of leaving Earth early).
Well... maybe, but I still think it's a reach. You'd think somebody would've mentioned any such Kzinti conflicts -- or that somebody, upon hearing of the Xindi attack, would've mistaken the name for "Kzinti." Maybe it's not an impossible fit, but it's an awkward one. Clearly the people who created ENT had no intention of making it compatible with "The Slaver Weapon," and if -- if
-- they had done a Kzinti episode in a hypothetical season 5, it most likely would've reinterpreted the specifics.
I really don't think its much of a reach. Yes, you would think there would be, but considering there wasn't even a mention of the Eugenics War until Season 3 or World War 3 or the post atomic horror until Season 4, I don't think it's much of a problem especially if you also consider how little those major events are mentioned in the other series. A Xindi/Kzinti mixup would have been funny, but unnecessary ultimately. Honestly though, I think part of why I don't see much of a problem is because I've never read any Known Space story and I see the Trek version as being very much separate and distinct despite the close adaptation of "The Soft Weapon".