Of the metals you mention, all have a melting point lower than iron except for titanium. Even titanium is not much higher than iron (1538 °C vs. 1668 °C), so I would expect that with a little trial and error, a Roman smith could melt titanium too.
You don't mention chromium directly (though it is used in stainless steel). It has a melting point of 1907 °C, this may put it out of reach as this is near the maximum possible temperature of a traditional forge (coal burns at 1927 °C under perfect conditions). Tungsten - not a chance at 3422 °C.
Romans smiths would probably take quite a while to work out proper techniques for steel, but it would certainly be within their raw capabilities. And steel would be the perfect addition to Roman metallurgy with its toughness and suitability to their existing techniques.
Working with titanium is considerably different than the other metals that the Romans used. As you try to form it mechanically, it becomes brittle quickly and must be re-annealed repeatedly as you continue to work it. Welding requires special methods (not that Romans would much care as rivets still work). Eventually Roman smiths might discover how to make some useful things from titanium. The big advantage of titanium is its low weight, but since Romans don't make a lot of airplanes etc. they might not find it worth the trouble.
Romans could not heat iron to the point that it would flow as a liquid (this is considerably higher than the melting point), but many bulk rearrangement techniques of iron working are greatly facilitated by heating to temperatures that are at least somewhat close to melting - as well as annealing and heat treating. Converting stainless bar stock into a shield by hammering at room temperature would be a monumental effort (if it could ever be achieved), but heating and hammering (including hammer welding) allows it to be feasible, and Romans were used to doing it for bronze. Unless the metal is quite ductile, a near melting temperature (near being a relative term) is required to form into arbitrary shapes.
I emphasized melting temperatures are these are a reasonable proxy for the ancient metallurgical techniques required. I.e., if the melting temperatures are approximately those of iron or less, they could soften and work the materials all other things being equal - titanium being an example of where other things are not equal.