In case you don't know, when "updating" Opera 15+ doesn't actually "update" your installation. Instead, it simply completely installs a newer version and tells the launcher to start launching the new .exe from then on. It's fine, but the old versions do not get removed. This is cluttering the system and is pretty much supposed to be unnecessary.

Can I safely remove those without having any impact on the latest version installed?

Yes, you can. You merely have to change the install path of either one.

I had Opera 12 on "Opera Presto" folder and Opera 15+ on "Opera Blink" folder. They work just fine