I am not sure about ancestry.com's guidelines, advice etc. But for genealogy in general, it is a commonplace that you always start the search with yourself, work backwards through your parents and keep going as far as you can. That's the only way you can avoid the problem you have already identified of mistaking someone with the same last name. And it does not even have to be what you'd think of as a common name now, like Smith. There was a pretty big family named Rumsey in the Colonies circa 1770, for example, and because the English had often very little imagination when it came to first names in the 18th c., some of them were named James. It's possible to jump to the conclusion that the inventor James Rumsey was simultaneously 1) a young man on the Chesapeake Bay 2) an officer serving in the old Northwest in the British Army and 3) a slave-holding merchant in New Orleans...but they were, of course, three different guys.

Geneological inquiries can cross borders- I know there are British , German and French family databases. But I've no idea if ancestry.com incorporates them.

I would suggest you x-post this to /r/Genealogy.