Next Step?

I have followed Steve Piatz's process for making a Lambic extract.
I brewed as normal and allowed the wort to cool naturally in my fermenter and sit in the kitchen for 2 days.
This morning I have full blown fermentation happening; without pitching my smack pack of Wyeast lambic blend and dregs of commercial sour beers that I have been building up.
Next step should I:
1. Add the smack pack and dregs as planned.
2. Add just the dregs.
3. Add some dry yeast and the dregs, save the smack pack for another.
The plastic fementer had been used for a sour before, it received a clean of boiling water and star san. Would there be a chance of the little critters still be living in the fermenter? And that is what is causing fermentation?
Ideas and thoughts?

The Steve Piatz method can make fantastic lambic. If you have a full-blown fermentation happening that quickly then you've managed to catch a lot of (possibly wild) yeast... or it may point to weak or intentionally low-effort sanitation processes and a chance of the little critters still living in the fermenter excitedly going at their new food source.

How is the aroma? That can be a useful indicator of the quality of yeast that's managed to take over your wort - if it smells like bread and/or fruit that's great, if it smells like baby diaper or band-aids that's not so great, unless you're into that. There's quite a bit of luck involved when it comes to wild yeast wrangling. You can catch good or bad without changing your process at all. Chances are it's leftover yeast from your prior ferment in that bucket if your cleaning regimen was less than rigorous.

If it smells good or at least not bad I would continue as planned (#1). Pitch the commercial culture and dregs. If it doesn't smell good I'd save those for another try and decide if you have the time and space for an experimental batch to mature. If no, try again. If you're bold, go for it. You could end up with a fantastic lambic-style beer if it works out.

One thing to remember is that the Steve Piatz method can allow enteric bacteria to play a significant role in the early stages of the fermentation (as it theoretically can in traditional lambic fermentation) - it can smell pretty unpleasant, but as the beer matures the Brettanomyces and other organisms will break down the really weird compounds into depth and complexity you can't get any other way.

Regardless, the one thing I personally would not do is leave this batch without any commercial cultures or dregs. The truly wild stuff very, very rarely ends up being something beyond an interesting oddity. The dregs and commercial cultures will help ensure that you end up with a beer that has at least some of the qualities of commercial lambics (particularly acidity) that you are most likely looking for.

I've actually got one going right now that I left to chill/rest open to the air for 48 hours, then in a carboy with no commerical pitches for another 48 with no signs of activity. At that point I added WLP655. Two weeks later it smells distinctly of wet soil, metal, and vaguely of mold. Hoping that what I said about Brett eating the bad stuff comes true on that one!

Good luck! These beers can be very rewarding.