-maille coif being too baggy
Common problem, it needs tailoring, just like fabric
-tunic being to baggy(one shoulder always wants to slide down)
Common problem, it should also be tailored. If not tailored, an easier adjustment might be to cut it down some at the top, add eyelets on the sides and lace it so that it is tight against the body. Most tunics (surcoats) seem to be rectangular, but should be flared from top to bottom.
Instead of |_| more like /__\
-we're my belt right(it always seems to high or too low and makes me look funny in my gear.
May be in part due to surcoat
-my helmet seems too small in width when if try wearing my arming cap with my mail coif.
Quite possible if it wasn't made for your head size
Should I wear my coif under my tunic?
Yes - and ideally underneath of the other mail. It makes it look like a single garment, which is more historical. Modern coifs also seem to be excessively long and wide, which throws the look off even more.
Should I get a better fitted tunic and helmet?
Yes, or adjust the ones you have (easier w/surcoat than helmet)
We're should I be wearing my belt? And does double wrap work better as far as making it fit goes?
Check historical pictures you want to replicate; look at their elbows. Standing straight up, your elbows will naturally sit right around your belly button. Use that to figure out where they have theirs at.
Would wearing my coif under the helmet to against medieval style and does
No, I'm not aware of any where it is worn outside of the helmet either