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I didn't really recommend against it. But I would have if you asked about converting a system beforehand. It's just a difficult job to completely flush on systems starting with glycol fluid. The two types do not mix... silicone will literally float on the other fluid, but technically they can coexist.

Funny you mention about the 'crusty' calipers. Most fail from the outside, not the inside. Dust boot damage during pad replacement (or worse, when the unknowing spray anti-squeal goo on the back of pads) allows water to get into the land just outside the main pressure seal. As water sits behind the dust boot and rust builds, it creates drag on the piston that prevents it from retracting normally. Not only does it make the caliper not function properly, but the abrasive rust often scores the smooth piston surface making it unusable.

The hydraulic fluid inside has no effect on this most common type of caliper failure.