According to White Labs' site, they say WLP650 was intended for secondary fermentation, but it would appear that if you use a yeast starter, you may be alright.
Q: I am planning a sour brown ale with bret bruxellensis as the only yeast. Is there anything special I should do with this strain and is this a good idea?
A. We don't recommend the vials for primary (not enough cells), but if you make a decent starter, it should be fine. You should plan on 3-5 days to grow up the starter, at least room temperature (but warmer is better) and aerate as usual. You are probably going to have to make twice as much of a starter as you normally would for Saccharomyces.