What if you just type the address without the ftp:// part?
Thanks for that suggestion because it worked. After a bit of testing, each time clearing browsing history, it appears:
ftp://+address ->google search for ftp://+address
ftp address only -> ftp site
usual web address with or without http:// -> website
Also suggested sites before pressing enter appear to come from browsing history, which is much better than I previously thought. I still suspect browsing history is going to google 'on the fly', not an issue if it doesn't include credentials like names and passwords.
So thanks for your suggestion, the bottom line I guess is hopefully the opera devs could check/ensure that such details can't be passed to any search engine.