Many companies that make solder and flux have useful guides for using their products. Here is one that covers soldering aluminum, stainless steel, and nickel: https://www.kappalloy.com/pub/media/pdf/Kapp%20solder%20instructions%20R6.pdf
These metals are a bit tricky to solder, but with the right solder, the right flux, and the right technique, it's totally doable.