Still seems to be some misinformation. My understanding on this very well-worn topic was that DOT5 absolutely is mixable with DOT3 or 4, since that is a legal requirement for any brake fluid. But just because you can, why would you - it costs more, and you wouldn't get the benefits like it being kind to paint etc.
Also, it's common to find the brakes a bit more difficult to bleed (i.e. start with a slightly more spongy pedal), but once properly bled should be absolutely fine.