Can DFU(OTA) only run on 7.0.0 version of the SoftDevice? thank you!
No, it works for older softdevice as well, just use DFU project from corresponding SDK version that support this softdevice. For example if you want to use softdevice v6 then you can use DFU project from SDK 5.2.0 - nRF51 SDK_v5.2.0.39364\Nordic\nrf51822\Board\nrf6310\device_firmware_updates\bootloader