So it turns out my Jirachi has a Lax nature (+Def -Sp Def). Eh, I guess it could be worse. At least Wish support Jirachi benefits from the defense boost, and I could always invest more EVs in Sp Def if need be.
My Rotom is Serious natured. I guess I'll just soft reset for a better one on my Diamond. I'll probably go with Rotom H, since Overheat is so useful (and because he handles Scizor so well).
So the team plan thus far is Roserade, Swampert, Rotom H, Jirachi, and possibly LO Jolteon (although I may potentially change this to Starmie, because one of my friends will be using a Forretress lead with Toxic Spikes). For the last slot, since most of the rest of my Pokemon are semi-bulky and will attempt to stall out my opponents while poison damage chips away at their health, I'd probably want a late-game sweeper/revenge killer, preferably on the physical side to compensate for my general lack of physical offense.