The answer is that it's possible to do them with lower resists, you just have to be extra careful about the huge damage spike attacks (anything ZK/SB do, Rakanoth's teleport move, butterflies, etc.), as they can kill you almost instantly if you're not at 100% health. Also be ready to kite Ghom if you're lifesteal isn't enough to negate his gas attack.
I've killed MP10 SK/Magda and Ghom/Rakanoth many times without ever having more than 600 AR, and I've had ZK dead and SB down to about 5% with 559 AR, (here's the gear build I used for that attempt http://d3up.com/b/1064197). I suspect they'll be a real challenge for you, but I'd also suspect they'd be a real challenge even with 600 AR.
what's the prefer build for uber solo? is it still nirvana?
I was under the impression that Nirvana does not play well with the uber fights, especially ZK/SB where there are no adds to utilize for infused with light. Both bells and tri-gen work fine for the other two duos. They work fine with ZK/SB too if you've got pretty good gear (KimSulki has videos showing kills with all sorts of fun specs), but the OP's gear might require a more specialized set-up. Earth Ally can be a huge help for this fight, as can SSS-pandemonium, dashing strike, serenity, or stun bells if you've got the mana regen to keep ZK locked down nearly constantly. No spec is going to make the fight ez mode if you're undergeared for it, but many have reported one or more of those skills got them over the hump.