The pH is fine. Generally fish will adjust to the pH level that the water is at so it's best just not to change it unless it's a huge problem.
As for the cory cat deaths...
How many have you had at one time? They need groups of 6+
What is your substrate? Cory cats need soft sand so they don't hurt their stomachs or barbels.
Were they getting enough food? Cory cats don't eat food like regular fish do. They eat scraps and food that sinks to the bottom, and won't eat flakes and floating food.
As for the guppy deaths...
I'm not really sure about this one. Were the platies getting all of the food before the guppies could get to it? They're not as aggressive for food as platies are.