Im playing The Dead Money DLC and so far ive found 250 Serra Madre coins.... Is it possible for me to leave the rest of the coins and finish the missions then go back to get the rest? Cause i need a gas mask "if there is one".... im not sure if i can even safely get the coins...

You can go back, but when you leave back for te mojave yyou wont be able to return. No gas mask.
Wrong section.

There's not strictly 500 coins in the DLC - which is what I thought at first.

You'll find plenty inside containers (dressers, garbage cans, etc.) aside from the ones you can find at the large fountains or the small wall-fountain-things.

You can also return pre-war clothing and cigarettes (packs and cartons) to receive more.. or just find the Sierra Madre snowglobe and get about 2,000 for it.

you need to search more...i got this just getting christine...think it was half way to her iirc

scrounge!

You'll find tons later on.