Sounds like playing to me!
Jackson can sound and look REALLY rough when playing. Lots of noises and growling and rolling on backs and biting at necks.
His "girlfriend" is Nala (she's about 6lbs, so less than half his size) and they wrestle, and Nala will roll on her back and put her legs up in the air, and Jackson is making growling noises and nipping at her neck.... and she loves EVERY minute of it and always goes back for more.
As long as no tails are down, or they are uncomfortable, afraid, yelping, etc, and keep going back for more.... they are playing. Dogs will be dogs but more importantly, terriers will be terriers.
Terriers definitely have a play style different from a lot of dogs.