One more thing! I cannot stress enough how much of an asset Luma is for Rosalina players. Seriously, Luma's pretty much your main source of racking up damage fast, and he's also your saving grace.
For RosaLuma players: Luma is extremely valuable! (almost more so than Rosalina herself). Don't let him die easily or get knocked out. You wanna keep Luma in there for as long as you can (just like Nana with IC's). If you aren't a very precise player, almost all of your attacks will hit or miss, but when it does miss, you can get punished very heavily whenever Luma isn't around. When Luma is around though, Luma can make up a lot of your mistakes by attacking faster, or taking hits for you. If you wanna get risky, you'd better do it quick and early in the game, or if your opponent is at a high percent while you aren't. That way Luma can respawn in your time of need if KO'd. Wanna get damage fast? Be with Luma. Wanna combo and punish? Be separate from Luma. Wanna KO? You can be together or separate from Luma (I'd recommend KOing with Luma for a better guarantee).
Fighting RosaLuma players: The highest priority on your list is getting rid of Luma. SERIOUSLY, KILL THAT THING AS FAST AS YOU CAN. I may seem like I'm exaggerating, but I'm not. Luma will be the biggest thorn in your side that you gotta take out. Trying to attack both of them can set you up really easily for punishment and being for being pressured. You gotta focus on one or the other during the beginning. Later on, if you think both are at perfect KO percent, then by all means, go ahead and go for the kill. Otherwise be patient. I can definitely see RosaLuma players being extremely patient as going in all out might not be the best idea for them. Being patient is key (mostly when Luma is there though).
This is all I got for now, but later on I'll say more on what I think.