Hello frozt, you've included a number of questions that can have some very lengthy explanations. So I'll give you a few brief answers and others will likely add their explanations as well. Regarding E7018, the description of the electrode, "low hydrogen", defines the characteristic of the electrode to exclude hydrogen from being introduced into the weld pool, this prevents underbead cracking, other additives that the fluxes possess allows hydrogen gas to escape from the weld metal while it is still fluid thus preventing the above condition and also preventing these gas bubbles from solidifying in the weld metal making it look like swiss cheese when it is cut, polished, and etched. The ac machine that you have now will not weld with E7018 electrodes, it will weld with a special electrode called E7018 AC, the standard E7018 is designed to be run on DC reverse polarity. It is also important that this electrode is stored in a holding oven after it has been removed from it's hermetically sealed package so that it will not pick up moisture from the atmosphere or other places.
     Scrap metal has inherent problems with identification. Spark testing can help you to determine mild steels from other possible materials such as cast iron, titanium, high-carbon steels, and such, but you're still not guaranteed that you'll have an absolute identification. Magnetism, weight, color, surface rust, and other characteristics will aid with identification of certain materials, but these are still somewhat subjective. Looking at the use of the material will sometimes give clues to it's type, a bed frame for instance is likely plain mild steel, your kitchen sink, if it is has a shiny, brushed surface, is likely stainless steel of some type of alloy. If your rowboat is made of metal and is light and easily maneuverable, it is likely an aluminum alloy of some type, marine grades are generally 5000 series. All of this may seem foreign to you, the best way to start figuring this out is to do some research.
     Gotta go for now, will add more as I have some time later. Regards, aevald

Back again frozt, anything that will knock or scrape off the slag will work. Some slags come off with more difficulty than others. In some cases how you have the amperage set will have something to do with how easily the slag comes off. Just make sure that you use good eye protection and also protect your body from the flying slag as it is still hot enough to give you a good burn. If you have the unlucky opportunity to end up with a piece of slag on your lip, up your nose, in your ear, or up under your safety glasses and on your eyelid or in your eye, you will definitely think real hard about how you clean off the slag.
     You ask about the weather, cold, moisture, yes these can have an effect on welding. Various grades and thicknesses of materials are susceptible to cold and moisture. That is why there are specific weld procedures for some materials, they may require preheats, interpass temperature monitoring, and postheat procedures, in some cases there are situations where humidity levels and rain can stop certain types of welding from being done. The average home builder will likely not need to be concerned with many of these types of conditions, however you don't know that for certain.
     As far as welding aluminum with your stick machine goes, I can't remember off the top of my head if the stick electrodes for welding aluminum can be run on AC or not. They do make aluminum stick welding electrodes and I have run some of them a long time ago. That particular way of welding aluminum might just about be the last way that I would like to weld aluminum. My experiences with that method weren't good.
     To sum a few things up for you frozt, possibly think about taking some welding classes at your local community college or trade center if you can, or possibly enlist the help of a welding friend. Also see about spending some time on either the Net or studying some welding books to expand your understanding of welding and it's related information. There are numerous sites dedicated to any number of topics that could answer your questions regarding rods, their uses and applications and also give you tips on understanding many of the ins and outs of welding. The forum itself has a search section that could bring up past discussions of some of the very questions that you have ask in your post and could also provide information on questions that you might have in the future. Good luck and keep us posted. Regards, aevald

1- already well covered.
2- As above the prior use will usually tell.  If you just want something to run a bead onto for initial practice it won't matter much anyway.  If you ask a welder he (she) will probably give you something to play with.  Could also be the beginning of a potential useful relationship.
3- Most of the 6011 sort of rods leave easily removed slag.  Most any hammer can be made to do.  The real thing is nice though,  a wire brush is good too.
4- Yes but for mild steel at the thicknesses you are likely to use it won't be a factor.
5- There are aluminum stick rods available probably some can be used with AC.  I have found them difficult to use but others do quite well with them.  Better for reasonably thick material.  Since the word decorative appears in your project description I would suggest that you try some other method of attaching these panels.  Pop rivets come to mind.

Come back and tell us how you do.
Bill