BT LE Connection Interval?
Can I assume that the connection interval is 120 ms?


No, the connection interval is 30 ms, but the device generates data in 120ms steps. You will see a Handle value notification only if there is new data.


  Is it common for 6 packets to be transmitted in such quick succession like this?


Look closer: It is 4 packets in rapid succession following by 2 more in the next interval. This is quite normal for BT LE connections. For example, Nordic semi NRF51 can transfer up to 6 packets in one connection interval (peripherial mode).