It will alter the balance, but that isn't necessarily a bad thing. Often the first pitch of those microbe blends leads to a very mild sourness, so stepping up the starter shouldn’t be a big issue. Don't go too crazy with the aeration, the lactic acid bacteria tend to be anaerobes. If you can get your hands on some good commercial sours I’d suggest pitching the dregs from them into the carboys as well. It may also be a good idea to add some dry yeast to help with the primary fermentation, my first batch of lambic took 4 days to start fermenting and never tasted right.
I recently filled a 60 gallon wine barrel we started with a microbe blend in 5 gallons of beer, a few weeks later we pitched 30 gallons of wort onto that and then 25 gallons the following weekend. Seemed to work fine, and the beer already has a nice sourness after just a few months.