For my tomme, which is either 2 or 3 gal/ batch, I add a pinch of B. linens and a pinch of Geotrichum candidium at the time that I add my starter. I allow the powders to rehydrate on the surface of the milk for 3-5 min, then stir them in. I don't premix with water.
I do the same with P. candidum when I make cheeses like Brie, etc.
I have never had an issue doing it this way.