I really wouldnt be afraid to use the foam. Its soooo much easier than the liquid. The liquid has a habit of running down your scalp, and you lose a lot that way. The foam is much easier to directly apply to the scalp. Longer hair can be tough, but just take the time to part the hair and get it...
Hey man,
Minoxidil still uses an alcohol base which is known to dry out skin. The white flakes are probably just skin getting dry. Try using a moisturizer a bit after the Minoxidil has some time to set it. Should help with the flaking, and it's more healthy for your skin anyway.