Just pitch it and wait, you may also consider pitching the dregs of an unfiltered commercial flanders such as monks cafe. You may want to taste it to make sure it is in fact getting sour and not just gross before buying a pack of Roselare but the infection looked a bit like lacto to me which would mean sour...Possibly really sour by the time the Brett character starts coming in after 6-12 months or so. If it gets too sour from this initial infection you can always blend it with a young beer to taste.
I had a rogue dead guy ale clone that had a weird grape/peach flavor that wasn't so great, not sour but it just tasted off so i inniculated it with a small sample from another sour beer I already had going over to it and now it is starting to taste much better, I think the flavors worked better in a sour beer.
Who knows maybe this beer will turn out better than you imagined. When homebrewing hands you lemons, make sour beer!