Well, it would depend on the layout of the coop and if you can block off part of it without inhibiting the function for the older birds. Coop looks rather small, but any way to fit a crate for the chicks into the coop?
Is the coop the blue building at the far left or the tiny 'coop' at the far right?
I like to brood chicks in the coop behind a temporary wire wall.
Much easier(after building the setup) than the 'wait until they are the same size' thing...