Do similar partners stay together longer?

Moderately yes. Similarity on values (kids, money, religion) and personality (emotional stability, conscientiousness) correlates with satisfaction. But similarity on all dimensions is boring. Most lasting couples are similar on core dimensions (values, attachment style) and complementary on others (one introverted, one extroverted).

Can opposites attract and stay together?

Yes, but it requires more work. Opposite personality couples are more interesting (different perspectives) but also more conflicting. They need to understand and respect their differences rather than expecting the other to change. The divorce rate for opposites is slightly higher, but many thrive.

What's the ideal similarity-complementarity balance?

Similarity: values, attachment style, life goals. Complementarity: personality (one detail-oriented, one big-picture), social energy (one introvert, one extrovert), skills (one good with money, one good with people). This balance gives you enough common ground plus enough difference for growth.