I have a bbc micro bit. It has a ARM Cortex M0 chip that supports BLE.
I want to be able to send messages via Bluetooth to this usb dongle (with only Bluetooth 2.0 support). Will I be able to send messages back and forth to the bbc micro bit with this usb dongle. Or is it the case that the bbc micro only supports BLE?
It does NOT support classic Bluetooth (v2). It is running a Nordic nRF51822 SOC, which can ONLY run BLE v4.1 full in central/peripheral/combined mode... as well as their proprietary radio waveforms such as ESB and Gazelle.
Bluetooth Smart is not backward-compatible with the previous (often called "Classic") Bluetooth protocol.
Thus, no, you would not be able to communicate with a Bluetooth 2.0 dongle. Doing so with a BLE-compatible device would require the device implement Bluetooth 4.0 with both the 'normal' Bluetooth and BLE portions of the standard.