You are right on your first 2 points, selecting birds for climate is most important as is positioning housing in the shade.
Electrolytes help but so do foot baths with ice in the water, ice blocks in the waterer (they don't drink much warm water), eliminate all heat generating scratch grains like corn and wheat (in summer I only give oats for scratch), a fan at the window to blow cool air into the coop at night or extract hot air, frozen milk jugs hung at roost height and misters if it isn't too humid in your climate already.