There are lots of feeding systems, previous posts show this, Braspenning fed Variamax all year round.
I think that as long as your birds have experienced all the grains you feed any changes or tweaks will be taken in their stride.
Remember that birds have favourite grains, when I look at their droppings I see multiple colours which indicates they do not all eat the food in proportion to the mixture. I guess the droppings would all be the same colour if only fed a single grain.