A good way to tell if something is hardened or not is to run a file across it. If it bites in it's not hardened if it slides across it like glass it is. Mild can be welded to stainless, but you need to use stainless rod. I use 316 rod 1/16" diameter for most things, but for something that small I just use 316 mig wire. One thing about Stainless is that it cannot be hardened like mild steel can it can be work hardened, but not hardened. Take stainless knives for instance they never hold an edge like a steel blade will.
Stainless it a very strange metal, it's soft yet hard at the same time and it's very gummy to machine, unless you use Carbide bits then it cuts like butter.
I don't see why I couldn't do it I make rollers for conveyors out of 11ga (.120") and they are always straight, and if they are not I nudge them into shape.

They usually turn between about 400 and 850 rpm and we use then for conveyors with a scale under it, and we weigh all the way down to a gram and any vibration gets transfered to the scale throws off the weight so I think I can handle it. Send me a sketch and I'll put it into Autocad to make sure it's exactly what you want.
You can usually find sst tube and rod in most hardware stores, along with brass copper and extruded tubes and such.
Jeff