It wouldn't be the end of the world.One other question for now before we get started with this new one. Should I use the same soil in the cup as the soil it will wind up in, or can they be different. What I was thinking was starting them in the Fox Farms, and then transplanting them into the build a soil blend.
Though I would suggest going with the same as the finishing soil.
Especially since you're doing a simple water only grow, it would be good for her to get use to the soil and all that it has to offer!
Also, FF can be a little hot for seedlings. Best to keep it simple and consistent!
Always remember K.I.S.S.