There is nothing wrong with soldering, as a means of improving a connection.
What matters is that the connection be electrically and mechanically secure - and we are not allowed to rely upon solder ALONE for the mechanical connection.
So, in the application you describe, the wires would have to at least be tightly twisted together before soldering them together.
Scotch 33? Well, given the choice, I'd prefer shrink-tube.
Also: Unlike in the 'old days,' you are still required to have the required pigtails / conductor lengths.