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.

Kymar is not for hydra, tag and seer nuking...

I figure he may be useful for stripping buffs until I figure out the mischief tank situation in my 2nd and 3rd Hydra team, but duly noted. 

He's in the second team with Seeker, and my Seeker is rather slow, just under 300. Kymar is the Aura lead in that team, so maybe with the Aura bonus and Seeker's Commanding Presence, Seeker is around 340 effective speed, but otherwise, he's not geared well enough. I'll try temporal chains for now. 

I do plan on pairing him with Seer in DT and dungeons. 

Would tend to agree. Could see him being fun in one of those obnoxious Trunda comps though for lower difficulties though