It’s possible that they are a mix of Rosgvardia and Russian regulars. I don’t remember Wagner operating T-72B3 tanks. They only operated a couple of T-90M’s. For all we know, perhaps some Russian regulars and Rosgvardian troops joined Wagner. It’s going to get confusing initially.
Best not to over think events as they unfold, that just leads to conspiracies blooming. As it stands, Wagner is moving into Rostov if they haven't already seizing the city. It's a long running public dispute with the Russian MOD that goes back to events during the Battle for Bahkmut and came to a head with the recent attempt to force Wagner to sign under the Russian MOD.
Considering the fact that we saw old T-55’s being pulled out of storage, civilian vehicles being used for logistics, and other unconventional equipment, yes it is very likely Russian forces that aren’t on the front lines are using trucks with mg’s mounted on them.
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So it looks like the tanks did belong to Wagner. That explains why the recent tweets showed them pointing guns at the building and stacking behind MRAP’s. I wonder if any Rosgvardia and Russian regulars joined them given how Wagner was primarily light infantry.