We dont work our horses in the heat because we dont have to. If your ride early enough or late enough, you can beat the heat. If you can avoid it, why wouldnt you? I know that some horses can go for hours in the 100 degree heat, but it is more about what they are used to than what they arent. On days like that, you want to take it easy and offer water whenever possible.
As far as the sweating, some horses just sweat more than others. I have a 'sweater' too and we just make sure he has a mineral block and lots of water.