i know this one may be obvious but im still kinda confused on why Ed got the nickname "Full Metal". can someone please help.

Good one me and my friend have been contemplating this for a while. I think that it was sort of a joke between him and mustang because he wasn't the one full of metal. Probably not the best answer

I think Ed can make anything into metal and if Roy is the Flame alchemist that means Ed IS the Metal alchemist but its true that Ed can make anything into metal

Take a look at his autmail.
What's automail made of?
There's your answer.
It could also be a joke that he was only part metal when Alphonse is literally full metal.

So, here goes nothing. Basically Hagane means steel so it should actually be full steel alchemist which could be due to his determination and stubborn attitude cause he was the youngest candidate and didn't deter from his goal. Secondly it could also be because of his ability to manipulate the earth and its elements which are mainly composed of the various metals like iron and stuff. I mean he can actually seperate it to its purest form. This is all based on the assumption that his limbs were not yet known to anyone other than Roy, Riza, Hughs and Tucker. Hope I helped. ^_^