I use Kymar on my second Hydra team, in Reflex gear. Without Reflex, though, I don't think he's particularly good in Hydra. You'd want to pretty much never use his A2 in Hydra - you only use it if you *absolutely* need to, because it'll result in Reflex potentially lowering the wrong skill CD. And as for the poisons - they're more of icing on the cake. His main job is to reset cooldowns. You'll want him on a team with Geomancer, or someone with a similarly important cooldown.
As for your second question, you're talking about arena I assume? Intimidating Presence overwrites Commanding Presence, even if the latter is higher level. Commanding level 5 is +15% to your aura, vs Intimidating level 1 is +2.5% / -5%, so you wouldn't want to take Intimidating. If your Arbiter is at least 340 SPD, then I'd probably go with Temporal Chains. Otherwise, I guess I'd take Brimstone, though in arena it's unlikely to really make a big difference for you.