Yes they are required by law to coexist without causing problems. The lowest boiling point fluid will control the boiling point, and as that's usually water in the hygroscopic glycol-based fluid. So cleaning out all old fluid makes sense from that standpoint.

Scare stories are perpetuated by the deceitful to scare the ignorant.

The major problem with the real DOT5 silicone fluid is that it entrains air and if you are not super careful you will end up with a spongy pedal. Yo have to be extra cautious to not shake the can or move it around and it should sit or days after being agitated. Rick Ehrenburg used silicone DOT5 in the Green Brick running the One Lap of America races, he would have noticed any problems with the stuff. I like it because it doesn't ruin paint. But I run mostly Wal-Mart DOT3 in my non-racer cars. So much for high tech!

R.