I would strongly suggest to AoE the adds instead of single target them, expecially if you have 3 offtanks in mixed gear (they don't really need it with that tactic to be honest).
Unless you are lacking locks/mages, that makes adds handling way more fast and allows more DPS on the boss and so reduces the chances of getting to enrage, and as well doesnt force you to chainCC last wave(s) of adds.
About tanking transition, our Phase Tanks usually wait until 100% mark comes, then start moving and move it about at same time that 250% mark. Then we get some seconds to get the add's and boss' attention to the appropriate tanks, then gather all on Hydross and AoE. Melee and single target DPS are on boss all time, but they are told to stop DPS when tank starts to move. DoTs are stopped at 100% mark (meaning that they run out between that one and the following one).
Shield Walling on frost phase is bad, just because the Water Tombs will kill more people at same time, while the poison will kill the raid 1 by 1.
Worlde - Prot Paladin
Darkworldie - Tank DW Frost DK
Uord - Prot Warrior
Huordie - Feral / Resto Druid