can the August Smart Lock use the hub in a Echo Plus which has a built in hub or do i need to use the August Connect Wi-Fi Bridge?
You'll need the August bridge, as the August connects via Bluetooth, and the hub in the Plus is Zigbee.
At this point you may be asking, "But don't all Echo units have Bluetooth?". Yes, but there doesn't appear to be any way to connect the two using that. Also, based on my experience with a friend's August lock, its Bluetooth range is quite short.