Will any ballast run any HPS or MH bulb?

Not always. Match the ballast's wattage rating to the bulb, and check whether the ballast is switchable between HPS and MH spectrums if you plan to run both bulb types across a grow cycle.

How often should I replace an HPS or MH bulb?

Most HPS and MH bulbs lose meaningful output after 10 to 12 months of regular use, even though they may still light up. Replace on a schedule rather than waiting for visible failure if yield consistency matters to you.

Does the reflector I choose actually affect yield?

Yes. A well-matched reflector spreads light more evenly across the canopy and reduces hot spots directly under the bulb, which affects both yield consistency and how close you can safely hang the light.