connecting multi-strand to copper wire for lighting

An obvious candidate for the "Darwin Awards"

. Doesn't sound like that much problem. I use wire nuts with a 'live spring'. The spring doesn't have plastic right behind it and deforms over the wires making a more secure connection (IMHO). If you take a wire nut off that has been tightened you can see the deformation. (Far as I know these don't exist in a size smaller than yellow.)

Use wire combinations the manufacturer indicates are in the capability of the wire nut. As suggested elsewhere I extend stranded wires slightly past solid.

IMHO 2 #12 solids and a stranded #18 is a bigger problem - the #18 can sit in the 'valley' between the solids. I twist the solids and keep the smaller wire straight.

A nice tight wire nut connection has always worked for me even with a mixture of solid and stranded wires and I've taken the connections apart several times afterward to make sure there was good contact area. I agree that pre-twisting the solids helps. In earlier days, I just soldered the bundle and put a wire nut over the top for insulation.

TKM