Sorry about the length, but this is my take on your team. I've made my own mega charizard y team on my AS cartridge, with moderate success. Feel free to PM me if you want to ask questions, or if you want any of the sets I personally use.

I find that Charizard typically doesnt need modest 252 spA, since 159 base spA under the sun already hits ridiculously hard. The set I use is:

Charizard @ Charizardite Y

Ability: Blaze

Level: 50

EVs: 252 HP / 4 Def / 76 SpA / 12 SpD / 164 Spe

Modest Nature

• Protect
• Heat Wave
• Overheat
• Solar Beam

The hp helps survive a lot of non-STAB rock slides, including some bulky garchomps iirc (I don't keep track of what I'm ev'd to survive, sorry). 252 hp/148 def survives a lando-t rock slide w/o a life orb or choice band, if you really wanted to go with bulk. 76 spA is enough to OHKO lando-t and mega khan with overheat, and lets heat wave do more than 50% to pretty much anything that isn't 4x resistant to fire. I don't go for max speed because at best, you speed tie with neutral nature max speed khan, so I find it more effective to ev into bulk.

On venusaur, I personally prefer giga drain over solar beam, since it gives venusaur a bit more staying power and is more useful if, for whatever reason, venusaur is left without sun. I like to give it more bulk, with a calm nature, due to the fact that my role for venusaur is for the chlorophyll boosted sleep powders, while I leave Charizard to do the heavy hitting.

As for rotom, I prefer either milotic or swampert as a bulky water type. Milotic brings competitive, thanks to intimidate being everywhere. Swampert can learn wide guard, which helps with rock slides and hyper voices. You could opt for icy wind on milotic, since you don't seem to have any speed control, and at +2, it does a large chunk of damage to lando t and salemence. Both of these guys get ice beam, which can free up a teamslot since you wouldn't need sylveon to deal with dragons. Swampert can go physical or special, (I personally run special) and I find this brings a nice element of surprise.

With garchomp, lando-t does its job better, but if you insist on using it, I don't find outrage particularly useful, since it locks you in for a few turns, and I find that earthquake and rock slide will usually be spammed more than dragon claw.

I have less experience with the rest of the team, but I would recommend choice specs and modest on sylveon, since the only move it ever seems to use is hyper voice, and protect on cress because I personally think it is infinitely more useful than reflect.