OK, a few definitions:
Yield strength is the amount of force required to cause the steel to yield, which means permanently deform (i.e. permanently stretch).
Tensile strength (a.k.a. "ultimate strength") is the amount of force required to cause the steel to actually break. This will be equal to or greater than the yield strength.
Minimum just means that the steel will be at least that strong.
Hardness is a measure of how resistant the steel is to scratching and denting. For structural usage it's probably not important, but would be important if you were looking for a durable finish, e.g. a workbench top or a tool bearing point.
Stiffness (you didn't ask about this, but it's another way of looking at the strength of a material) is a measure of how much something deflects when you put a force on it. Steel alloys tend to be pretty similar in this regard.
As you can see, "strongest" doesn't really have a specific definition, it depends on what you're looking for.
Here's an analogy for the difference between yield and tensile strength:
Imagine you have a spring. You pull on it a little, and when you let go it returns to its original shape. This is "elastic deformation", and no damage has been done. Now you pull hard on the spring and it doesn't return to it's original shape anymore. The material has yielded and you have "plastic deformation". This may or may not be considered "failure", depending on the application. Now pull really hard and the spring breaks. That's the ultimate strength. Clearly the spring has failed now.
As for the ranges: "steel" is a non-specific name for several alloys and it can be made in several grades, hence the ranges you've found. The material is usually designated with an alloy number. "Cold rolled" and "hot rolled" are methods for shaping the steel, and don't really tell you anything about the strength.
I should also point out that all of these properties that I've mentioned are for the steel material itself. If you want to know the behavior of an actual piece of steel, you need to know both its material and it's shape.
You're right about the arc welding full-face mask - they use very dark glass to accommodate the bright arc. There is no arc here, so in such mask you'll see nothing. Also unlike arc welding (where you spend bit of time aligning, then you lower the mask and then spend considerable time doing the actual welding) here you spend almost all the time on aligning the material but the actual welding takes only tiny fraction of total time spent working (real spot welders use capacitors to do the job in sub-second timeframes. This is where the savings come from). So a vision-limiting full face mask won't work here, because you'll have strong incentives to simply not wear it.
I believe you should use gas-welding goggles, the convertible kind where you can lift dark glass and work with clear glass. Just make sure they are rated for operation with the clear glass alone, because cheap models have only superficial clear plastic that won't protect against larger projectile or UV. And of course, stay clear from "steampunk", "mad scientist", etc decorative goggles because they're intended for nothing more than, well, decoration. Especially now, around Halloween, the market is flooded with such toys.
Best Answer
Well.. stainless steel pretty much welds just like regular steel (in so much as its a pretty simple welding procedure)
Galvanised steel, (which is regular steel coated in a Zinc like covering) is the monster when you are welding.
so, the answer is, that really its pretty easy to weld the two, you could use regular metal filler rods to weld the two, and it would bond pretty well.
the problem comes when you think about your application (why are you welding the components) -- both stainless and galvanised are rust proof/resistant when they are not damaged (by welding) -- when you weld galvanised metal, you need to prepare the weld area by grinding off the zinc surface and exposing the raw metal, only then can you weld... -- now that area is not protected by the zinc and will rust --so if you wanted a rust free application, you have messed up the surface at the area of the join.
Stainless steel, can be welded with a stainless steel rod/filler, which will not rust, however.. when it gets too hot, or blends at the weld surface to the regular metal, it too will start to rust at the weld site (very slightly).
You also mention welding machine... well, this also depends on thickness of material and frequency of task, you can use MIG, TIG, and ARC welding to do this task... and they will be proper welds (the MIG and TIG would handle thinner material welds) and they will be proper welds. As mentioned, the only thing that is not proper, is that the site will now rust.
NOTE: The response is a DIY level response. If you are performing engineering grade welds for high stress scenarios, you would not really use these two materials together in the first place... But when you have the metal or the need.. one makes a plan.