Welding Tips and Tricks

I have dabbled in home building, maintenance, and a variety of types of projects over the years. It was always frustrating when I needed to weld something and could not. After retiring from an engineering career I took the basic (32 hours) stick class at the local community college. Teacher gave us just a peek at torch, and tig. I spent some time designing my first project. More on that later. And spent some time watching videos on Utube. It seemed to me MIG was the lower cost and versatile way to go for what I invisioned for my welding hobby. I caught a deal on CL for a CH farm hand 110 vac 135 mig unit that worked. No gas bottle or wire. I got a roll of Flux core and set about to try it. No scrap metal! I've been collecting nuts and bolts all my life but not steel scrap. I finally found a spare trailer hitch and scraped off some paint. I got a good cold weld to a big flat washer.
So, I have a couple of beginner basic questions
If I go slow and hot can I melt anything to stick to 1/4 plate? Budget is an issue so I won't have a gas bottle for a while. I'll look at the salvage yard to get a bigger power/extension cord and a bigger ground lead.
If I do get a bottle, is 100 % CO2 , since it gets better penetration, going to be a problem for other work?
Can anyone tell what to look for in old gas bottles that makes refillers not want to service them?
Next I need to clean out the garage to have a place to work this winter. Who knows, I might find some scrap metal.
OK I got some 3/4-3/4-1/8 angle from the scrap yard for .25 per pound. Now....
look forward to sharing with you folks coming up.

welcome to the forum.