i am a former professional welder-there are myths and sayings but heres the facts from the oregon structural enineers union-in my words

yes there are fumes-weld in  a well vented, or fan blown area

you can weld structurally if you remove the galvinized coating at least 1/4 inch from the weld surface

if this is done on both surfaces-strength loss is minimal if present at all

and in my opinion, if you have to do it, so be it-but non galvanized is easier

yes it will hold paint if prepped normally-although it will always look like ass IMHO

i used to weld galvy plate to galvy pipe for pile driving type situations, then houses sat on em-so yes it is plent strong- but if you try to weld without removing the coating- it will leave voids, pockets of gas and air, and not pennetrate well, resulting in a poor quality weld


hope that helps