Salon downstairs, apartment upstairs. Why do the leases look so different?

Normal, yes. Residential leases are short, standardized, and heavily regulated, and the landlord pays taxes and insurance out of the rent. Commercial leases get negotiated between two businesses, so the costs get pushed around, and a common structure has the tenant paying base rent plus a share of taxes, insurance and common area maintenance. Pro rata share usually means their square footage divided by the building's, so if the salon is 1,000 sf in a 2,000 sf building they'd owe half. Read that clause carefully, because how the denominator is defined decides how much they actually pay.

On the empty salon question. That's the real risk in a two-tenant building. Your income drops by whatever the salon paid, immediately and completely, and salon space usually has plumbing and electrical put in for that use that the next tenant may or may not want. Ask how long the current tenant has been there and what the remaining term is.