Is Highland romance the same as Scottish historical?

Yes — the terms are interchangeable. "Highland romance" specifically connotes medieval or early-modern Scottish clan settings.

Where should I start with Highland or Viking romance?

Julie Garwood's The Bride or The Secret for classic Highland; Diana Gabaldon's Outlander for time-travel Highland epic (the TV show adapts the novels faithfully); Kathleen Woodiwiss's The Flame and the Flower for an older-school sweeping-historical take. Modern Viking romance is sparser — Johanna Lindsey's classic catalogue remains canonical.

How spicy is Highland/Viking romance?

Generally more on-page than Regency — the wilder setting carries looser conventions. Gabaldon leans explicit; Garwood mid-range; newer indie Highland/Viking runs the full spectrum. Check spice ratings per book.