First thing I'm noticing about your team as a whole is that you want to revolve your team around Milotic's Rain Dance. That is not a bad thing but in order for your team to thrive while being "themed" is that it needs more of the required tools to do so, i.e. you need more Rain Dance users. The weather no longer stays forever, you only get it for 5 turns (8 if the user is holding Light Clay).
Now on to your team (apologies for the long intro)
Milotic:
Milotic looks good, maybe not the best nature but personally I would prefer a neutral nature over one that hinders a key stat. I would switch the Speed EVs for HP EVs. and combo that with Leftovers and Aqua Ring.
Helioptile:
The nature you are looking for is Timid (+Spd - Atk). I personally like him, I do know others who disagree, but hey, not their team ;). I think with the right tools he could pull off the role you are looking for, sure maybe others can do better, but it's nice to see some diversity. (Rambling sorry)
I would change his EV spread to 252 SpA / 252 Spd / 4 HP and either Scarf him or give a Life Orb. The rain should balance out the Life Orb.
Yveltal:
For him I would lean more on the Special Attacker side of the fence so you can get more out of Hurricane (It's a special attack and you are digging the rain). Nature for him I would say Modest (+SpA -Atk) and EV spread him 252 SpA and either 252 Spd or 252 HP, your choice. As far as his moves I would swap Dragon Rush and pick up Dark Pulse for a second STAB and drop Phantom Force for Rain Dance.
Blaziken:
He is a good sweeper but due how common he is, he is very predictable. If you are planning to Baton Pass with him I would throw protect on him so you get a free Speed Boost to pass. I would pass the speed to someone who would abuse it a bit more than Ferrothorn though. I would pass it to Helioptile or Yveltal.
Ferrothorn:
Ferrothorn is a killer wall, He tanks a hit while dealing damage with Iron Barbs and Rocky Helmet.
I would go with Leech Seed over Ingrain.
As for your final slot:
If you do want to go with the Rain Dance team then you would want to reserve this slot for one more Dancer. One of my Favorites lately (Along with many others lol) is Azumarill. It's tanky and it hits hard. Other options are Goodra, Jellicent, Jolteon, and Vaporeon.
All in all your team looks good, but it does need some work. Just find what you want to do and build around it. I hope this answer helps, and good luck with your team.
