A somewhat older furnace, cast iron piping, gas fired furnace.
When a pipe connection is loosened there is always very rusty water inside. Told by an 'expert' that it is normal to have/leave rusty water in the pipes/rads, & not to remove/drain out the rust. Shouldn't this be done. Won't the pipes rust even more, & cause blockages? There was a blockage recently, the rads all went cold, & the tech. said a ball of stuff like mud, came out. Eventually the blockage loosened & the heat started up again. How do we drain out excess rust?

I do not suggest you do anything. That system needs a major flush and a chemical treatment. This work should be done by a professional since it is possible too much action to the pipes can cause some leaks. It sounds like professional maintaince has been put off for way to long.