On small maps I usually go for a knight overrun, where as on large maps I either go for cavarly (upgraded from horsemen) or even tank overrun. In then end I am always using some sort of conquest, though I will still build a spaceship once I have reduced the computers to 1 or 0 cities.
As far as mountains go, I was unaware of the mountain ratio being higher on small maps, but in either case 2 move units are still the most powerful and best option in my opinion. I usually have 2-3 ships land at different points on an island adjacent to cities and quickly take those cities in the same turn as landing, so the 2 movement is critical.