Will I need to put air stones in my freshwater tank if I had overflow/sump filtration?

Answer 1

You shouldn't need one. As long as there's somewhere in your system that creates some splashing (usually bottom of an overflow box) or preferably, a ripple movement across the surface of the water (from the return) you'll have enough movement to add and circulate dissolved oxygen.

Answer 2

No
You will get more than enough oxygen in your water using an overflow to a sump.