My conclusion on Gravity and RainDance in conjunction.

That's because they're stall teams. They're supposed to do that.

Sweepers that rely on DD only need 1. 2 if they're the bulky variant. 4 is overkill.

Gravity should be used more, though.

I disagree with DD'ers needing only 1 DD to sweep a team. Usually, if I use DD, I try to get 2-3 in for OHKOs. I usually set up my teams to predict status infliction (as you've noticed, I'm sure). For Salamence, this isn't the case since his ATT stat is extremely high and 1 DD is enough to wipe out more than half a team, that and along with STAB'd Outrage. IMO, personal preference, but when I spam Curse, DD, or SD, I try to reach the limit to ensure that I can wipe out most of the opposing team. Though if I know for sure I won't survive the upcoming attack, I'll immediately stop and start attacking.

Gravity is great. I stop Bronzongs from abusing that Levitate ability and start spamming STAB'd EQs on them.