Solid rain team. Rain teams are definitely going to go through a lot of changes in the near future because of the Tornadus-t ban. You've definitely got a good one so far, but the metagame is probably going to shift before you can really perfect this one. I think you could really benefit from some set changes without even having to change any Pokemon.
First, I want to recommend Timid 252 Spe / 252 Sp Atk Choice Specs Politoed instead of the one you currently have. It goes against the conventional wisdom, but I've been using it a lot lately. I have not looked back since. Part of the reason that Max Speed Politoed is so effective is its surprise value. Your team is ideal for it too because you can nab surprise KOes against Pokemon like Rotom-w and Dragonite because they expect to outspeed. You've got a lot of Tyranitar and Hippowdon checks, I don't think you'll miss the extra bulk too much. Conserve Politoed and conventional Sand counters like Rotom-w will have trouble taking 2 STAB Rain Water Attacks. I also like Surf instead of Hydro Pump for the accuracy, but that's up to you. You can also consider Ice Beam if you want to hit Celebi, but Psychic is effective against Toxicroak as well.
Furthermore, I think Double Dance Thundurus-t is a better option than Expert Belt. I personally think Expert Belt is a bad set, but your team could use raw wall breaking power. Right now a simple Skarm + Bliss core could wall your entire team, even playing conservatively. Defensive teams are definitely on the rise, and Nasty Plot Thunder will plow through them easily. Offensive teams also have a very difficult time responding to Agility, and the combination of Agility + Nasty Plot is basically impossible to counter.
Use 248 HP and 8 Spe for your Scizor. It's the usual Scizor set, and there's no reason not to use it. At the very least, you can Speed tie with other 8 Spe Scizor, which as I mentioned, is the most popular set by far.
On Gyarados, I have to recommend Sub DD Bounce. It's a great, old set that doesn't get enough love these days. Use 56 HP / 248 Atk / 204 Spe for your EVs. There isn't a lot to say about the set except that it's definitely the best one. Sub allows you to beat Status users and weak Scald users. Fast Politoed will be a great partner for Gyarados because you can weaken Rotom-w, Gyarados' number one counter. Gyarados is a fantastic wall breaker, and it should be your main weapon against Sand teams.
Finally, I want you to use Trick instead of Psychic/Energy Ball on Latios along with a Timid Nature. You never know when you'll need to Trick a problem Pokemon like CM Jirachi, Blissey, or Ferrothorn, and Timid allows you to beat Scarf Keldeo. Scarf Keldeo is a pretty important Pokemon to beat since it's very popular on good teams.