-ATK +SPD Zelgius or +HP -RES?

Use the later. I'm guessing at some point, you'll give him Bold Fighter, which makes speed irrelevant while making Atk the most important stat for him.

The plural of (1) Zelgoose would be (2) *Zelgeese*

I'm of the opinion that there's no bad boon and no good bane on Zelgius, only preferences, but all of them are workable without too much difficulty. Even 24 res turns into something meaningful once the distant def seal gets involved, and if we start adding ward armour? I myself have +res -HP and I actually don't like it. It's not so easy to panic ploy now that I removed his summoner support.

+HP -res is just very normal, does stuff very normally.

-atk +spd is more if you want to play him like a non-armour and a duelist that just has a lot of stats (especially HP and def), an absolutely devastating special, and adds utility via innate distant counter. Since he has a 16mt weapon to compensate for having an atk bane, he's basically a significantly better version of statlines that look like 32/32/32/16. Builds that interpret him this way might look something like:
Swift sparrow, atk/spd bond, or brazen atk/spd or atk/def (fury is also reasonable but I don't like the recoil when he has 38 def and will probably survive more than one fight)
Renewal or vengeful fighter (warp powder is fine too)
Panic ploy
close def, distant def, QR if you're not using vengeful fighter, whatever.

If you're going to give him bold fighter without something like death blow or brazen atk/def, I probably wouldn't lose his atk. The spd can be useful on enemy phase for blocking doubles, but I'm not convinced it's worth the atk bane.