your post shows your thinking, and thats good.
Dont use trade names for soft solders, use the technical name, for
example lead/tin 60 /40.
Next, google for soft solder specifications, theres many different
sorts and specifications. The more you read up the better you can
For base as in brass copper tin plated steel etc these soft solders
are just fine, tho joint construction is different to silver brazing.
Also you need to make sure you use the correct fluxes for the metals
you plan to join. Always experiment on scrap to get the technique
right before you put your production pieces together. Saves you a lot
of wasted time and effort.
As to where to buy soft solders, the thin types are used in
electronic component assembly. down to 1mm or less. Do you have a
Tandy store near still? They stock electronic soft solders. Check them
out. Or find some one who does pcb repairs.
To give you an example, I minted a single sided 1in dia coin in 1/32
brass of a character like one of Snow whites dwarfs.
I wanted to make it double sided. so I used some 1mm electronic
solder flux cored around 1.5 in long coiled in a spiral.
placed it between the 2 coins, held it with tweezers in a soft
propane flame and the 2 fused together without any excess.
This was an order for a show where the organisers wanted 500 to give
to all the children. It was called “Sweinegeld”.
One parent said you will always have this money in your purse! I made
most of them on site.