if you have an ac/dc stick welder that is the best thing you can use to learn. They make a filler rod for pretty much any kind of metal you can think of. It is easier and more important to watch the puddle while stick welding so you can really see what is going on and figure out what is good and what isnt. You can do quite a bit more with a little mig welder than what most people say/think, alot of it has to do with prep and technique. Although the applications of which you can use the welds you made with the little welder a bit limited. The best way to learn metallurgy is to either take a class or do lots of reading if you are good at learning things on your own. You could also post on weldingweb as they have tons of info about pretty much everything you would ever need to know.
when it comes to welding everybody knows something somebody else doesnt, whether it is true/safe varies from person to person but some things are proven facts that dont really change. Some people say you cant mig "cast iron" and others say you can do it just fine. The truth is that there are different types of cast iron and some you can weld with damn near anything and others you might as well use super glue because it will hold just as well.
listen to people, check facts, try some things for yourself and learn from all of it.
