I have used Automec silicone fluid in all my vehicles for a number of years now.
Here's Automec's description of their product.
"Unlike standard, glycol based, clutch/brake fluid, Automec DOT 5 - silicone fluid is non - hygroscopic which means it does not absorb moisture. This will help prevent corrosion in the system and, under heavy braking conditions, it tends not to boil. Also, it won't harm paintwork if spilt.
NOTE; It is completely safe to mix Automec DOT 5 silicone clutch/brake fluid with standard, Glycol based, clutch/brake fluid. However, in order to obtain the maximum performance benefits, it is best to drain the brake system refill with Automec Dot 5 silicone fluid."
Ideally the system should be clean but if you drain out the old fluid first then bleed the silicone fluid through thoroughly you should be fine.