I'm really not sure about adding Volbeat. The niche as a Prankster Pokemon is there, but it does recieve a lot of competition from RU's other Prankster Pokemon and Baton Pass users that makes me a little bit shaky on it all.
When it comes to Baton Pass, Volbeat recieves stiff competition from Nastypass Togetic, who doesn't have Prankster Encore, but has a ridiculous amount of bulk and something even more important: no priority on Baton Pass. This way Togetic can pull off a Baton Pass and get its recipient in safely most of the time, something Volbeat really struggles with, and considering you can only have one Baton Pass Pokemon on a team now i'd almost always go to Togetic before Volbeat for my special attack passing needs. Gorebyss and Combusken are two other important Pokemon to consider in the Baton Passing role.
When it comes to weather setting, you still run into a problem. Liepard, Meowstic-Male, and Whimsicott still accomplish the job better. Whimsicott's great typing gives it more opportunities to switch in and set up Sunny Day (it doesn't get Rain Dance), and all the other relevant things weather Volbeat wants, such as U-turn, Prankster Encore, or Prankster Stun Spore. Along with some other goodies such as Tailwind and Memento (have fun vs +2 Victreebel x__x) Whimsicott might be better off running an offensive set, but that's a moot point when even the inferior defensive set is doing the job better than Volbeat :(. Whimsicott can also set up on the things galbia mentioned above thanks to its typing and more, at least for sun teams.
When it comes to Rain Dance, there's Liepard, who isn't quite as bulky but still has access to all the important support moves weather Volbeat wants (Encore, Twave, U-Turn), and also possesses STAB Knock Off and a decent Attack stat to hurt things if it needs to.
Lastly, boy Meowstic gets both Rain Dance and Sunny Day, it doesn't have access to Encore or U-turn, but it still has Thunder Wave and something else really nice: Dual Screens, which are really helpful in getting things like Shell Smash Omastar and Growth Victreebel set up and ready to go.
Aside from doing the same thing Volbeat can do but arguably just as effectively, none of these Pokemon are weak to Stealth Rock, while Volbeat is.
TL;DR, not sure about Volbeat, the other Prankster Pokemon seem to simply do the job better on paper, and when it comes to baton passing nastypass Togetic gives it stiff competition especially given the BP limit.
As for Tangrowth, i'm not directly opposed to it, but i'd like to point out that while it recieves competition from the other Grass-types: it's no Florges VS Sylveon or Jellicent VS Slowking. The defensive set still has higher all around offensive stats, Leech Seed, more physical bulk, and Knock Off to help seperate itself from Amoonguss, and Tangrowth's immense bulk, access to Sleep Powder, and Regenerator make an offensive set quite viable without being totally overshadowed by Sceptile or Mowtom, so keep that in mind when discussing its rank.