Wait 12-24 hours between adding the Campden and adding the yeast. That will stun the native yeast that is present and then start to dissipate so the cultured yeast can take off and dominate the fermentation.
K-sulfite effectiveness decreases as pH decreases, but the yeast will have problems if the pH is too low. The lower limit is somewhat dependent upon yeast strain, but you don’t really want to get below 3.1 (where wine yeasts start to have problems) or above 4.0 (where the campden becomes ineffective). Cider typically falls in the middle of that range, so no worries.
I would only add acid if you need it for taste. Some apple varieties don’t have enough on their own, in which case you can add a little malic acid. That is something I wouldn’t due however until just before you are ready to bottle.