Nice team guys, but I think you'll struggle against a well-played Ferrothorn on an offensive rain team. Whenever Kingdra comes in after an opposing Thundurus-T/Tornadus-T has nabbed a kill on something, it will be met with the aforementioned Ferrothorn, who it cannot do anything to. Ferro resists both of Kingdra's STABs, breaks its substitutes with power whip and then leech seeds it. You say in your threat list that Heatran walls Ferro and threatens an OHKO with fire blast, which is true, but it's taking sr + leech damage as it switches in, and the opponent can make the easy switch to Politoed; rinse and repeat later when you try and revenge kill their Thundurus-T/Tornadus-T with Kingdra. Your own Thundurus-T is a good backup check to these sweepers but Volt Switch and a Choice item means it will be switching a lot; coupled with an SR weakness, it'll be worn down fast.
So, I think it would be best to try out a defensive Starmie. Not only does it spin away stealth rock with ease, which will make Thundurus-T + Scizor volt-turning incredibly hard to stop, but it also lures in Ferrothorn and cripples it with a scald burn, which means Kingdra can beat it; with the attack drop, Ferro can't break your sub with power whip while it's losing health each turn, meaning you can alternate between sub and dragon dance until you have a +6 Kingdra behind a sub which is pretty much gg unless they have a Quagsire who still won't be appreciating a rain-boosted waterfall. As for who to replace, I'm thinking Celebi, since Starmie has similar resistances to it and therefore your synergy'll remain mostly the same.
Starmie @ Leftovers | Natural Cure
Timid | 248 HP / 44 Def / 216 Spe
Scald | Psyshock | Rapid Spin | Recover
Cool team, hope this helped.