If you want to hit hard, bring whichever element you have the most out of in that rainbow team. Your soul + one Hime doing good damage should be way better than everyone hitting like a wet noodle.
They will, starting from Second Anniversary update (Feb/April, depending on what Nutaku feels like).
You get them from the (true) Rag Disasters, although, I hear they are rare as fuck. Also, while technically they do boost all characters, to do their thing you need to have 5 or more of the same type of weapon in your Grid. For a F2P, this basically only means Thunder, since you'll have 4x Pride Guns from Union Events alone. For every other element, you'd need to bring SRs...
Well, I guess if your aim is to have a rainbow team, you can just bring SSRs of the same type from multiple elements.
Thank You very much for information!