A thorough flush is a good idea. Although most brands of antifreeze will be compatable with others (meaning they won't turn into a slug of goo), mixing different types does minimize any advantage of any particular brand.
All the major antifreeze brands have products compatable with aluminium. I use Havoline or Prestone because I can get them anywhere.
The bleeding procedure is described in IAP's tech page:
bleeding the cooling system.
To flush I drain the system then use lots of water (plain old tap water is fine for flushing) with the drains open. Then I close the drains and fill with distilled water (you can buy it for about $1/gallon) - use the bleeding procedure - and run the engine until it is warm enough to open the thermostat and circulate the clean water. Let it cool and drain that out. Do that once or twice more until what drains out is fairly clean.
Finish by adding the specified quantity of coolant and top up with distilled water. Bleed the system and you'll be all set. If you drain and re-fill every couple of years and use the same product to re-fill you won't need to do such a thorough flushing every time.
BTW, here's a tip for cleaning out the coolant reservoir: rinse it out then put in a handful of crushed ice. Shake that around inside the reservoir - the ice is abrasive enough to scour the inside without harming the plastic reservoir. Then rinse it out with hot water and the ice and scum will come out easily.
BTW #2, it is also a good idea to remove the radiator and take it to radiator shop (look for an 'old fashioned' shop that actually works on radiators vs a shop that just replaces them with new). They can either chemically clean & flush the innards or open it up to physically scrape it clean ("rod it out').
P.S. the Sicilian is describing the block drain (behind the exhaust manifold). The bleed screws are on the top of the water pump and the top of the intake plenum.
