What's the gravity on the sweet beer? What yeast strains did you use? How long has it been fermenting? Fermentation temp?
I would say adding lactic acid is a quick band aid at best and I don't think you'll be happy with the end results if you were looking for a sour beer...patience is critical with sour\brett beers.
Primary: Sour Brown
Secondary: West Flanders Red, Golden Sour, Saison Brett #2 with Brett custersianus and Brett naardenensis, Saison Brett #1 with Brett bruxellensis
On Deck: 100% Brett IPA, West Flanders Red #2 with ECY BugFarm