...there was this one time I tried to play as HC on Earth18...
Oh yeah. I've played that one several times, only on Noble difficulty though.
The Inca have a unique situation, because they get a continent all to themselves, but they can't really access it until they research sailing. If you play your first hundred turns right though, space/time/culture victories are possible. Regardless, this is what you need to do:
-Quickly tech masonry and build Great Wall
-Then, tech sailing and get a sailboat right away.
-After a certain pop (Can't remember, would have to reattempt this one), keep training settlers and transporting them to the other parts of the continent. Greater emphasis should be made on settling the coasts. The Great Wall ability means that you can completely neglect your military for a while. Get at least five other coastal cities in South America.
-You need iron working eventually to make use of the grasslands under that jungle.
Several goals you should have in mind:
-Make sure your borders cover the south american border, so that no one else can settle in south america.
-Train at least 20 workers to clear the jungle.
The start will be slow, but after around 1400 A.D, your tech rate should be sky high from all the cottages you should be working. You could probably discover optics before Old World civs.
I would say that the worst starting point in 18-civ Earth would be the Americans. The Aztecs are just very aggressive.