I have some hard cider I'm making, and yesterday afternoon after I'd pressed all my juice, I put it in my sterilized carboy, and added the recommended (per the bottle) amount of potassium metabisulfite. Today, as I was preparing to pitch my yeast, I noticed that some natural fermentation has already started. Should I be concerned about this, or just go ahead and pitch my yeast as I normally would, and just hope/assume the much greater numbers will outcompete whatever natural yeasts were still left in there?
Pitch away. Sounds like maybe there wasn't enough metabisulfite but it should all work out. I'd do a strong active pitch of yeast. If you don't have that and were pitching dry I'd go with two packages per 5 gallons or so.
FWIW, some of the best ciders I've had were fermented naturally with what came on the apples. Its also made some pretty bad cider to, you just have to ride it out. Pitching your own yeast now will hedge on the side of success.
You should only be concerned if you're going for a specific flavor profile and are mostly relying on the yeast for such.
I recently made a batch of cider with a full load of wild yeast (cider kept unpasteurized and no chemicals added) and regular ale yeast added. During the fermentation there was a bit of competition between the different yeast strains which was noticeable in the trub as well. The final product ended up with a sour, tart flavor and slightly lower ABV compared to a full ale cider I made in parallel as a control batch. Your mileage may vary with the wild yeast as the breakdown of different strains on the apple skins will vary based on your location.
I personally think the wild yeast adds a good character, but it does make the results hard to reproduce. If you want to avoid the wild strains, I would suggest boiling/pasteurizing the cider before you pitch the yeast to try and eliminate the other microbes.
I know many wine and cider brewers like to use NA/K bisulphite in brews. I have no real idea why one would use it at the start of a brew as (in my experience) it reduces all fermentation including that of the introduced yeast as well. One might use it at the end of fermentation to reduce/stop further biological action ("as a preservative") once all required fermenting has been done - but I still don't agree it is necessary. Bisulphite is not the most useful of chemicals for a human to ingest and I would personally shy from using it in any foodstuff. If one must use something then I would begrudgingly advise using potassium sorbate. But in preference I would use nothing and let the natural process complete naturally. If one is really convinced that unwelcome wild yeasts or bacteria (eg lactobacilius) are present then I would recommend the "correct" method - pasteurisation at 60 degree Celsius- to remove or reduce their activity.
However having added the bisulphite there is no reason why one cannot add more yeast to see if it improves the situation. Its just that the added yeast might itself be stymied in action by the added preservative. As apple cider is normally fermented and conditioned for a long time I would add yeast, leave it for some months and then re-investigate to see what happened.
However for the next brew I would recommend pressing and lightly (gauze) filtering the juice and then adding an active yeast solution quickly thereafter. I would not add any preservative at all but if I did it would be at the end and not the start of fermentation.
But whatever, I hope the cider turns out well.