According to what I just read, you can't mix DOT 5 with anything else, as it's silicone based, as opposed to glycol based. It was developed for use in applications that may have high moisture intrusion, such as military.
To confuse things more, DOT 5.1 is glycol based, so could be mixed with DOT 3 or DOT 4.
I would flush everything out and use DOT 3 or DOT4. Even my wife's '13 Escape specifies DOT 4. I'm don't know of any vehicles that specify DOT 5.
The latest isn't always the greatest.
This is an interesting read.
