Thinking about a second dog — what mix of breeds/sizes actually works well together?

The opposite-sex guideline has some basis but it's not a rule — same-sex pairs work fine with the right temperaments. More predictive factors: energy level match (within a range, not identical), age gap of 2+ years so the resident dog isn't overwhelmed, and the new dog's socialization history. With a gentle female Golden, a male dog with a calmer temperament and similar or slightly lower energy works well. Avoid very high-prey-drive breeds if she's calm — the mismatch creates stress.

Consider fostering before adopting for the second dog. It lets you trial the dynamic before committing and you'll immediately know if the energy match is right or wrong. Most rescues allow this and it's the single best thing I did before adding my second.

The herding breed + retriever thing is breed-dependent. My friend's Golden does great with a Aussie mix. But a high-drive herding dog with strong instinct to chase and nip can stress a gentle Golden who doesn't understand what's happening. If you want a herding dog, look for one with lower drive or an older, calmer individual.