Good overview. I agree with pretty much everything you said. The only thing I'm not sure about is whether Surrakar is better than Selkie, but same as you, I'm no expert on Fish. Oh, and also, Tajuru is solid SB hate for Stax, but the question is whether or not it's better than what we already have.
Surrakar Spellblade is interesting. I think the lack of evasion hurts it a little, so if you're looking to draw an out to a freshly-tinkered robot, he won't help you there. Same goes in the mirror for fish; I'd rather have a Selkie that gets through most of the time versus a Surrakar that will just trade with whichever blocker gets left up. Still, if you can get it out early and a counter war happens afterwards, or you have some broken blue cards to cast, then he's your man.
I don't like Tajuru in Fish. I think it's a sideboard card that is only good against 1 card in 1 deck, versus something like Nature's Claim which is much more versatile against not only Stax, but the rest of the field as well. I'd much rather drop a land and destroy a Crucible while my Stax opponent has a Strip Mine/Wasteland in the yard, compared to holding onto the Tajuru Preserver. That Nature's Claim will also destroy Smokestack, Tangle Wire, Lodetone, Sphere, Chalice... plus Oath, Time Vault, Painter, Grindstone, and others I'm sure I missed.
Then again, my Fish build doesn't have Nature's claim in the side anyway...
