Plenty of people have mixed flocks, its how you end up with backyard/barnyard mixes.
Pecking orders are strange thing - but the ugly duckling of a certain fairy tale is a really simplistic, often inaccurate way of assuming flock dynamics. I'm constantly adding new birds every three weeks +/-, so the dynamic in my flock is rather fluid. If there's a hard and fast rule, it has yet to reveal itself...
and no, a DBrahma isn't "too big" for an EE. Your hens will put their keel on the ground when they squat and distribute all his weight for the brief moments he's on her. DB are slow growers. Hatchery stock, I'd be surprised if you had a male over 8.5# - 9# in the first year. The EE hen will likely be full grown in the same time period, likely close to 5.5#, maybe 6# depending on mix.
I had a 13.8# CornishX Roo mounting a 5# Comet (RSL) at 9-10 months. Fat ^&%*^& couldn't successfully do the deed, but not for lack of trying. No injuries to my little layers.
Where injuries do occur is overmating - when either due to a lack of recpetive hens, or some favoratism on the Roos part, only a couple girls get most of the attention. That's very hard on the feathers.
...and if you had ducks, the drakes are VERY fond of grabbing the hen's neck to stop her from running away. Bald patches, even with a ratio of one drake to 4, 5, 6 hens, aren't completely unheard of.
