The Gauss warthog, and the Rocket hog are available at levels 7 and 6 respectively. However, they are severely outgunned by the scorpion tank. I suggest that they are given armor boosts, as they get destroyed very easily at the level 6/7 Req stage.
I got a Legendary level 6 Urban Rocket Warthog, it was gone in less than a minute.
When I’m a gunner I always alert my driver to keep circling around the tank.
With the Gauss cannon its only an east 4 - 6 shots to take them out.