If there were hundreds of appearances afterward of the Metal Men/Atom team, then yes, that would be their first appearance.
No it wouldn't be. You don't even know if a team exists at this point, let alone if they have a name or whom the roster consists of. Best watch yourself, Sir. You are making a very strong argument for the 60.
That's where we fundamentally disagree, as I've said repeatedly in this thread. I don't care when they get a name, and I don't care if we know a team exits at the time that they first gather to defeat a bad guy. Their first adventure is their first appearance.
That might be called the first Metal Men / Atom team-up. However, if they later form a team (with a name obviously) then that would have another first appearance.
Larry Page and Sergey Brin might have sold road-side juice together years before Google exited, but Google was not founded until Google actually got founded - regardless of whether those two had worked together earlier or not.
Similarly, some of the people in TT (not even all of people who would later found TT was there) could have worked together earlier than founding TT, but TT was not founded until it was actually founded... which did not happen in 54, but rather happened 'after' that event. Therefore the first appearance of TT logically happened in 60.
By the way, the interesting thing is that 54 is actually the origin of TT. The events that took place in 54 prompted them to form TT. Chronologically having the origin before the foundation is like these things happen in real life. However, in comics we have gotten used to this sequence very often being the other way around. A group is introduced, and later the origin (which obviously happened before the foundation) is explained.