So here's a chart with the change I made to the team. The Electric weakness is mistaken as you have Thundurus-T as a resist, but the chart doesn't factor abilities.
I'll also repost your pokes with some new sets that I think may work better.
So the last spot there with the blank picture is Thundurus-T.
Politoed, you have an interesting set. However I think we can change it to better assist your team. As you can see here in this picture, Poli's stat are more geared for SAtk or SDef, although variations have been used, including Defensive Toed.
For Poli I will give you the Defensive variant as Ferro can run Special Defense.
Politoed @ Leftovers
EVs: 236 Def / 240 HP / 32 SAtk
- Perish Song
- Ice Beam
Scald has a chance to burn which cripples phyisical attackers. Ice Beam is to hit grass types who switch into Scald, or dragons. Protect is for stalling while Perish Song forces switches or causes KOs. 240 HP gives you an HP stat of 381 which is perfect for lessening the damage of moves that wear down your HP, allowing some EVs to be placed into Attack for a bit of a heavier hit.
Ferrothorn is so nasty to deal with in Rain. For running Defensive Toed and as shown in a few, SDef Vapo, I think we can increase your offensive presence on the team. A strong set to run on Ferrothorn is BandFerro
Ferrothorn @ Choice Band
Trait: Iron Barbs
EVs: 252 HP / 252 Atk / 4 SDef
- Gyro Ball
- Power Whip
Double STAB with Bulldoze and Explosion to round off the set. Gyro Ball with the nature is going to dent anything with base 80 speed or more. Power Whip is other STAB and with base power of 120 it's going to dent anything that isn't resistant. Now Heatran can easily switch into either of these STABs so with a bit of prediction go for Bulldoze and destroy it in one brutal hit. Explosion works incredibly well for destroying anything that doesn't resist and denting several who do resist.
Vaporeon is also heck to deal with in rain due to its ability Hydration. You can run a SDef set to balance out your Defensive Poli.
Vaporeon @ Leftovers
EVs: 252 SDef / 252 HP / 4 SAtk
- Hydro Pump
Toxic/Roar is just plain annoying. Rest heals you (duh :P) and Hydro Pump hits incredibly hard in rain even with only 4 EVs. Fully Special Defense. Not too much else to say, don't stay in to take physical attacks, you won't survive.
This thing is beastly in rain. I would suggest a Sub Bulk Up set which I hate dealing with in rain. Works very well as late game clean up.
Toxicroak @ Black Sludge
Trait: Dry Skin
EVs: 252 Atk / 252 HP / 4 SDef
- Bulk Up
- Drain Punch
- Sucker Punch
Standard set, Sub, Bulk Up, Drain Punch for STAB and Sucker Punch for Ghost and Psychic types. He regains all of his health in one turn. Not much else to say, keep the sub up, he can't take many hits.
This guy can wreck havoc in rain with STAB Thunder. Your set would work if I hadn't suggested a subset set for Toxicroak. So we'll change it a bit. And you need a Scarfer anyway.
Thundurus-T @ Choice Scarf
Trait: Volt Absorb
EVs: 252 Spd / 252 SAtk / 4 HP
- Focus Blast
- Grass Knot
- Hidden Power [Ice]
The set provides perfect coverage. Grass Knot absolutely destroys Gastrodon and Jellicent (although you can Abuse Thunder for that). HP Ice is for Dragons and Grass types. Focus Blast for dem Steel types who dare switch in for whatever reason. Special Attack is a brutal 427 with Scarf boosting speed to around 450. Works very well as late game clean up or to create dents in the beginning of the game.
Last but not least, your new team member. Latias who synergizes pretty well with the rest of the team. There are two sets you can run. A Specs set and a CM Set. I'll post both and you can try them both, see which you prefer.
First, the CM set.
Latias @ Leftovers
EVs: 252 HP / 4 SAtk / 252 Spd
- Calm Mind
- Dragon Pulse
- Hidden Power [Fighting]
Set up a sub, set up and begin sweeping. HP Fighting hits both Tran and Ferro and also Skarm who tends to be Defensive > SDef. Dragon Pulse hits everything hard after a few CM and is your main STAB attack.
The second set, Specs, abuses Rain and hits very hard.
Latias (F) @ Choice Specs
EVs: 252 Spd / 252 SAtk / 4 HP
- Draco Meteor
- Hidden Power [Fighting]/Psyshock
DMeteor is STAB while Thunder and Surf abuse rain. HP Fighting is only there to hit Ferrothorn. Ummmmm yeah that's about it. Latias has great bulk to abuse so both sets utilize it for survival then smash back with unriddled power. Psyshock is there for secondary STAB and if you want to hit Blisseys. With it however you are walled by Ferrothorn. Either one can work.
Lol this took way longer then I thought xD I kept changing things. One thing that was entirely removed was hazards. Now I forsee a lot of hell for that but in all honesty most teams carry a Spinner and they just help to achieve KO's and not much else. The other thing is the fact that your two walls are water types. This shouldn't be too much of a problem as you can switch to Lati and Ferro who both have bulk to absorb some hits.
Good Luck and I hope this helps your team,