I do Gatorade, too, because I only do it now and then.
Worming with chemical wormers is a point of contention on this board. Chickens are going to have a few worms (you and I probably do, too) and they are going to have some mites and lice. It's a matter of degree of infestation, plus overall health of the bird and their resistance. For me, I'm going to go with a twice a year program, I believe. I just did ivermectin and will probably use a different one in the spring, to avoid resistance. I agree, wait til it cools off, unless you think one is in trouble with them; gives you some time to research it here and think about it.
The Activia would be fine, it's what I use because it's in the house (yes, we buy the plain,) but whatever you want. They only need about a tsp. each; they don't digest the milk well, so you're only after the organisms in the yogurt.
I have a couple of Wyandottes. All mine seem about equally stressed by the heat, and all pant. The coop is very open air (hardware cloth for parts of the walls on all 4 sides) and I keep a fan on; they stay in the coop in the heat of the day, for the shade, and I hose the outside of the coop down (metal) once or twice on sunny days.
There are always wild birds around here, and I know the older flock had them once, so I don't even check for mites and lice, I just treat them with pyrethrin/permethrin dust every couple of months, and spray nests and roosts and such with Adams flea and tick spray. I also sprinkle DE around everywhere anyway, about once a week, and that does seem to help control the numbers. I have to do the checking alone (well, I could recruit my son, but haven't) so I just used a flashlight and looked at the vent area, the one time I did it. I think I did turn her over to do it. I didn't check all of them closely; I found some critters and eggs on a cooperative hen or two, only had a few then, and just started treating. Once in a while I will feel like invisible fleas are jumping on me, and that's when I remember it's time for pyrethrins again....
I don't really know what's up with the Wyandottes hanging their butt feathers down. You might be approaching an egg issue, they might have all gotten into something moldy, they might be loaded with worms.... I do not know.
I'm going to PM you, too.