Any mod that sees this can close the topic. I figured out what's going on:
Looking at the /var/lib/ambari-server/data directory reveals a "version" file. This was reporting 1.3 to me on the slave nodes (ones running a DataNode and JobTracker). Given that the error above says the server version is 1.3, that matched up and made sense to me.
It turned out that ambari-agent and ambari-server were *actually* 126.96.36.199-1, and this was revealed doing a yum list installed | grep ambari
So once I realized this, I was able to downgrade the new node and it added to the cluster properly. This caused a new problem, but that problem is not relevant to this thread.