I use power poles for 99% of my stuff. Power poles has the "clips" for varying size wires. They use "AMP" rating. A 30 amp clip is solid. A 45 amp and larger clip are open and need to be crimped, then soldered (Some of the suppliers say they don't need to be soldered - I solder mine anyway.)

The external plastic housings are the same for the majority of the sizes.

As mentioned above, a GOOD flux is needed (a good flux removes/neutralizes any contaminates and allows a good bond). One of the best fluxes I have found is the liquid flux that comes with "Stay Brite Silver Solder".