Would it be ok to solder,brass or hard metal, to make table legs? Or Would the joint not be able to hold much weight?
A good quality "turbo torch" or an oxy-acetylene torch is needed for brazing. Also, you will need the correct type of brazing rod matched for the material you are brazing. A good brazed joint will take practice. It is not as easy as soldering
Solder? Very unlikely. The bond is designed for electrical conductivity not physical strength (or, in the plumbing case, to achieve a certain strength via capillary action in a very small space). But brazing, that could work, if you have the capability to heat the metal, such as a forge. And welding of course is as strong as the metal you are welding itself.
If you are building workbench table from construction steel, e.g. square tubing, stick, MIG, or TIG welding is the way to go.