1. Not all of them must directly benefit from rain. There are other kinds of teammates you can consider: Steel-types and Bug-types, that appreciate the weakened fire attacks, grass resist, and for extra coverage. (Scizor, Aegislash, Metagross, Mawile are some examples.)
2. I would prefer Leftovers on Blastoise over the Mega Stone, considering its build and how you aren't really taking advantage of Mega Launcher. Normal Blastoise should also have the Hidden Ability for maximum effectiveness. For other megas you can use, I listed two already in item #1. You can also try Mega Aggron.
3. I don't like Stall Ludicolo... I'd prefer it to be a Swift Swim sweeper (or maybe replace it with Kingdra, for the same Swift Swim). Also, why are you using the weak "Mega" Drain when Ludicolo can have "Giga" Drain.