It's certainly tight, six minutes is the official minimum connection time at Ely, question is whether the train from Cambridge arrives at Ely on the same platform as the departure from Ely onwards to Peterborough. Some do, some don't. Not sure that the trains on your itinerary actually do. A transfer from platform 1 over to platform 3B, if that's what's required, might be an issue if carrying heavy luggage etc., given it's such a relatively tight connection.
P.S. Aren't itineraries from Cambridge through to Peterborough not possible that don't require a change at Ely? 1 train per hour?