You could introduce black if you want.

I've combined brushed nickel, aged steel and black fixtures in my open concept and they don't fight each other.

That is fine, I would probably stay away from gold tones such as brass.

I second painting the curtain rods. My husband has painted several light fixtures, towel rack, TP holder and a pole lamp in oil rubbed bronze, and they came out beautifully. We went with Crylon paint as the reviews for Rustoleum talked about the nozzle being defective in a lot of them. I estimate we've saved around $400 doing this instead of buying new lights and other fixtures.

I would apply the same rule I do with mixing any colors in a room.... I like them to all coordinate but not necessarily match.