Depending on how you do it, you really don't even need resist for your add tanks. The way we did it was two MT's (one at any one time) and 2 offtanks. At any one time, we have 1 tank on Hydross and 3 on the adds. The three tanks pull the adds to Hydross, wait about 10 seconds, and then the locks and mages unleash AoE. The adds aren't alive long enough for resist to be a problem (at most we have 2 people in resist, usually one or none), and the tanks are stacked on top of each other so PoM, Chain, and CoH can easily keep them up. Plus the enrage timer is a non-issue, even when we first started we had 2-3 minutes left on enrage when he died.